@@ -0,0 +1,34 @@
+---
+'@objectstack/spec': minor
+'@objectstack/rest': minor
+'@objectstack/runtime': minor
+'@objectstack/cli': minor
+'@objectstack/lint': minor
+'@object-ui/app-shell': minor
+'@object-ui/i18n': minor
+---
+
+feat(spec,rest,cli): a doc book's title and section titles are translatable — `books.<name>` in the translation bundle
+
+Clause-②: yes (widening)
+Metadata-protocol: added
+
+A package that ships its docs in two languages showed a half-translated manual: each doc's body followed the reader's locale (`doc.translations`, `<name>.<locale>.md`), but the book title and every sidebar section title stayed in the authoring language. The book's own inline `translations` map was retired in 17.0.0 because nothing read it, and the translation bundle had no key for a book.
+
+- **`@objectstack/spec`:** `TranslationData` (and the registered `translation` item and the platform bundle, which share its groups) gains `books`, keyed by the book's `name`: `label`, `description`, and `groups.<key>.label` for each group by its stable `key` (strict: any other key is refused; `name` / `title` point at `label`, and `sections` / `chapters` / `toc` point at `groups`; the singular group spelling `book` points at `books`). A new translator, `translateBook`, overlays them and is registered in the metadata-document dispatch, so `GET /meta/book` (list and item) is served in the request's locale. A key the bundle does not carry keeps the authored string. The rejection for the retired inline `translations` map on a book now names the bundle keys to move the strings to.
+- **`@objectstack/rest`:** `GET /meta/book/:name/tree` translates the book it resolves through the same dispatch before rendering the spine, so the tree, the book list and the console's docs sidebar agree. `MetaBookTreeSources` gains `resolveI18nService()`, which both transports supply.
+- **`@objectstack/runtime`:** the dispatcher's book-tree route supplies the same i18n service, so it answers the localized tree `RestServer` answers.
+- **`@objectstack/cli`:** `os i18n extract` scaffolds `books.<name>.label` / `.description` and `books.<name>.groups.<key>.label` from the stack's `books[]`, and `os i18n check` / `os lint` coverage report an untranslated one as `i18n/missing-book`.
+- **`@objectstack/lint`:** `translation-target-unknown` resolves `books.<name>` against the declared books (a sibling package's included, and a package id for its implicit book) and `books.<name>.groups.<key>` against that book's group `key`s, so a mistyped book name or group key is an error instead of a translation nothing reads.
+- **`@object-ui/i18n`:** the console's language loader recognises `books` as a translation group, so a served document that carries only book translations is namespaced like any other.
+
+Example (`zh-CN` bundle of an English-authored package):
+
+```ts
+books: {
+  crm_manual: {
+    label: '使用手册',
+    groups: { start: { label: '快速上手' }, pipeline: { label: '销售管道' } },
+  },
+},
+```
@@ -406,8 +406,8 @@ Three rules the shape enforces, all of them closed since #4001:
 
 - **The top level is the declared group set and nothing else** — `objects`,
   `picklists`, `apps`, `messages`, `globalActions`, `dashboards`, `datasets`,
-  `reports`, `pages`, `flows`, `positions`, `permissions`, `metadataForms`,
-  `settingsCommon`. A group invented beside them
+  `reports`, `pages`, `flows`, `positions`, `permissions`, `books`,
+  `metadataForms`, `settingsCommon`. A group invented beside them
   (`"account"`, `"fields"`, `"list"`, `"industries"`) is rejected **by name**
   at both authoring doors, with the group to use instead named in the
   rejection.
@@ -496,7 +496,7 @@ what the overlay writes back as the node's `title`.
 The contract is `t(key, locale, params?)` — the target locale is always passed
 explicitly, and the key itself is returned when no translation is found (there
 is no `defaultValue` option). The key is a **dot path into `TranslationData`**,
-so it starts at one of the fourteen declared groups.
+so it starts at one of the fifteen declared groups.
 
 ```typescript
 const i18n = kernel.getService('i18n');
 
@@ -1783,6 +1783,7 @@ The published metadata item body, opaque by ruling (1C). Shape is the item's own
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 | **settings** | `Record<string, { title?: string; description?: string; groups?: Record<string, object>; keys?: Record<string, object>; … }>` | optional | Settings manifest translations keyed by namespace |
 
 
@@ -47,7 +47,7 @@ const result = BookSchema.parse(data);
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
 | **name** | `string` | ✅ | Book name (namespace prefix recommended, like every metadata name) |
-| **label** | `string` | optional | Display title |
+| **label** | `string` | optional | Display title — translated by `books.<book>.label` in the translation bundle |
 | **description** | `string` | optional |  |
 | **slug** | `string` | optional | Portal URL segment; defaults to name sans prefix |
 | **icon** | `string` | optional |  |
@@ -67,8 +67,8 @@ const result = BookSchema.parse(data);
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
 | **key** | `string` | ✅ | Stable group key (used by overrides, deep links, explicit `doc.group`) |
-| **label** | `string` | ✅ | Section title — first-class, i18n-homed |
-| **translations** | `never` | optional | [REMOVED] Inline `translations` on a book (and on a book group) was removed in @objectstack/spec 17.0.0 (ADR-0049) — no resolver ever read it. The book tree endpoint and the docs portal render `label` / `description` verbatim in every locale, so a localized book shipped its authoring-locale strings to every reader. Delete the key. NOTE the near neighbour that DOES work: `doc.translations` is live and read on every doc render path — localize the docs themselves, and the portal picks the reader's locale up from there. This edit left `os migrate meta` with the protocol-16.x conversions: apply it by hand, or run the tool on a release that still carries the conversion — https://objectstack.ai/docs/protocol-upgrade. |
+| **label** | `string` | ✅ | Section title — first-class, i18n-homed: translated by `books.<book>.groups.<key>.label` in the translation bundle |
+| **translations** | `never` | optional | [REMOVED] Inline `translations` on a book (and on a book group) was removed in @objectstack/spec 17.0.0 (ADR-0049) — no resolver ever read it, so a localized book shipped its authoring-locale strings to every reader. Delete the key and move each locale's strings into the translation bundle: `books.<book>.label` / `.description` and `books.<book>.groups.<group_key>.label` (`os i18n extract` seeds them). The book list, the book tree and the docs sidebar resolve those per reader locale. A doc's own title and body still localize through `doc.translations`. This edit left `os migrate meta` with the protocol-16.x conversions: apply it by hand, or run the tool on a release that still carries the conversion — https://objectstack.ai/docs/protocol-upgrade. |
 | **order** | `number` | optional | Order of THIS group within the book |
 | **include** | `string \| { tag: string }` | optional | Rule that derives membership (glob or tag) |
 | **package** | `string` | optional | Scope the rule to a package id (default: the book package; cross-package via ADR-0048) |
@@ -115,8 +115,8 @@ Type: `'public'`
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
 | **key** | `string` | ✅ | Stable group key (used by overrides, deep links, explicit `doc.group`) |
-| **label** | `string` | ✅ | Section title — first-class, i18n-homed |
-| **translations** | `never` | optional | [REMOVED] Inline `translations` on a book (and on a book group) was removed in @objectstack/spec 17.0.0 (ADR-0049) — no resolver ever read it. The book tree endpoint and the docs portal render `label` / `description` verbatim in every locale, so a localized book shipped its authoring-locale strings to every reader. Delete the key. NOTE the near neighbour that DOES work: `doc.translations` is live and read on every doc render path — localize the docs themselves, and the portal picks the reader's locale up from there. This edit left `os migrate meta` with the protocol-16.x conversions: apply it by hand, or run the tool on a release that still carries the conversion — https://objectstack.ai/docs/protocol-upgrade. |
+| **label** | `string` | ✅ | Section title — first-class, i18n-homed: translated by `books.<book>.groups.<key>.label` in the translation bundle |
+| **translations** | `never` | optional | [REMOVED] Inline `translations` on a book (and on a book group) was removed in @objectstack/spec 17.0.0 (ADR-0049) — no resolver ever read it, so a localized book shipped its authoring-locale strings to every reader. Delete the key and move each locale's strings into the translation bundle: `books.<book>.label` / `.description` and `books.<book>.groups.<group_key>.label` (`os i18n extract` seeds them). The book list, the book tree and the docs sidebar resolve those per reader locale. A doc's own title and body still localize through `doc.translations`. This edit left `os migrate meta` with the protocol-16.x conversions: apply it by hand, or run the tool on a release that still carries the conversion — https://objectstack.ai/docs/protocol-upgrade. |
 | **order** | `number` | optional | Order of THIS group within the book |
 | **include** | `string \| { tag: string }` | optional | Rule that derives membership (glob or tag) |
 | **package** | `string` | optional | Scope the rule to a package id (default: the book package; cross-package via ADR-0048) |
 
@@ -185,6 +185,7 @@ Platform translation data — the per-app groups plus the platform-only `setting
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **metadataForms** | `Record<string, { label?: string; description?: string; sections?: Record<string, object>; fields?: Record<string, object> }>` | optional | Translations for metadata-type configuration forms keyed by metadata type |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 | **settings** | `Record<string, { title?: string; description?: string; groups?: Record<string, object>; keys?: Record<string, object>; … }>` | optional | Settings manifest translations keyed by namespace |
@@ -295,6 +296,14 @@ Translation data for a single object
 | **label** | `string` | optional | Translated permission-set label — overlays the set's own `label` |
 | **description** | `string` | optional | Translated permission-set description — overlays the set's own `description` |
 
+### Nested Shape: `PlatformTranslationData.books[string]`
+
+| Property | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| **label** | `string` | optional | Translated book title — overlays the book's own `label` |
+| **description** | `string` | optional | Translated book description — overlays the book's own `description` |
+| **groups** | `Record<string, { label: string }>` | optional | Group (section) translations keyed by the group `key` |
+
 ### Nested Shape: `PlatformTranslationData.metadataForms[string]`
 
 | Property | Type | Required | Description |
@@ -344,6 +353,7 @@ Served translation data — the platform groups the browser reads, without the s
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 | **settings** | `Record<string, { title?: string; description?: string; groups?: Record<string, object>; keys?: Record<string, object>; … }>` | optional | Settings manifest translations keyed by namespace |
 
@@ -453,6 +463,14 @@ Translation data for a single object
 | **label** | `string` | optional | Translated permission-set label — overlays the set's own `label` |
 | **description** | `string` | optional | Translated permission-set description — overlays the set's own `description` |
 
+### Nested Shape: `ServedTranslationData.books[string]`
+
+| Property | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| **label** | `string` | optional | Translated book title — overlays the book's own `label` |
+| **description** | `string` | optional | Translated book description — overlays the book's own `description` |
+| **groups** | `Record<string, { label: string }>` | optional | Group (section) translations keyed by the group `key` |
+
 ### Nested Shape: `ServedTranslationData.settingsCommon`
 
 | Property | Type | Required | Description |
@@ -564,6 +582,7 @@ Per-app translation data for objects, apps, and UI messages
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **metadataForms** | `Record<string, { label?: string; description?: string; sections?: Record<string, object>; fields?: Record<string, object> }>` | optional | Translations for metadata-type configuration forms keyed by metadata type |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 
@@ -673,6 +692,14 @@ Translation data for a single object
 | **label** | `string` | optional | Translated permission-set label — overlays the set's own `label` |
 | **description** | `string` | optional | Translated permission-set description — overlays the set's own `description` |
 
+### Nested Shape: `TranslationData.books[string]`
+
+| Property | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| **label** | `string` | optional | Translated book title — overlays the book's own `label` |
+| **description** | `string` | optional | Translated book description — overlays the book's own `description` |
+| **groups** | `Record<string, { label: string }>` | optional | Group (section) translations keyed by the group `key` |
+
 ### Nested Shape: `TranslationData.metadataForms[string]`
 
 | Property | Type | Required | Description |
@@ -743,6 +770,7 @@ One locale of translations — the `translation` metadata type
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **metadataForms** | `Record<string, { label?: string; description?: string; sections?: Record<string, object>; fields?: Record<string, object> }>` | optional | Translations for metadata-type configuration forms keyed by metadata type |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 | **locale** | `string` | ✅ | BCP-47 locale this item translates (e.g. "zh-CN") |
@@ -862,6 +890,14 @@ Translation data for a single object
 | **label** | `string` | optional | Translated permission-set label — overlays the set's own `label` |
 | **description** | `string` | optional | Translated permission-set description — overlays the set's own `description` |
 
+### Nested Shape: `TranslationItem.books[string]`
+
+| Property | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| **label** | `string` | optional | Translated book title — overlays the book's own `label` |
+| **description** | `string` | optional | Translated book description — overlays the book's own `description` |
+| **groups** | `Record<string, { label: string }>` | optional | Group (section) translations keyed by the group `key` |
+
 ### Nested Shape: `TranslationItem.metadataForms[string]`
 
 | Property | Type | Required | Description |
 
@@ -302,11 +302,34 @@ expands to *the rest* — every doc the group's rule would have claimed but that
 names, in `order`-then-label sequence. An entry can also be an object to attach a `label`
 override, a `badge` or an `icon`, or to point at an external `href` instead of a doc.
 
+#### Translating the spine
+
+A book's title, its description and each group's section title translate through the
+translation bundle, like an app's label and its navigation labels — keyed by the book's
+`name` and each group's `key`:
+
+```typescript
+translations: [{
+  'zh-CN': {
+    books: {
+      crm_docs: {
+        label: 'CRM 文档',
+        groups: { overview: { label: '概览' }, guides: { label: '使用指南' }, admin: { label: '管理' } },
+      },
+    },
+  },
+}],
+```
+
+`GET /meta/book`, the book tree and the docs sidebar serve the reader's locale; a key the
+bundle does not carry keeps the authored string. `os i18n extract` seeds the keys, and
+`os i18n check` reports a missing one as `i18n/missing-book`. The docs themselves localize
+through `doc.translations` and `<name>.<locale>.md` variants.
+
 <Callout type="warn">
 Inline `translations` on a **book** or a **book group** is rejected: no resolver ever read
-it, so a localized spine shipped its authoring-locale strings to every reader. The near
-neighbour that *does* work is `doc.translations`, read on every doc render path — localize
-the docs themselves.
+it, so a localized spine shipped its authoring-locale strings to every reader. Move those
+strings into the bundle's `books` group above.
 </Callout>
 
 ### How `doc` and `book` compose