diff --git a/static/js/core/types.js b/static/js/core/types.js
index 030f29a3..58c87cf8 100644
--- a/static/js/core/types.js
+++ b/static/js/core/types.js
@@ -313,3 +313,45 @@
  * @property {string|undefined}    [next_cursor]  - Absent when the last page is reached.
  */
 
+/**
+ * @typedef {Object} ContactEmail
+ * @property {string}  email
+ * @property {string}  type        - e.g. "work", "home"
+ * @property {boolean} is_primary
+ */
+
+/**
+ * Mirrors the backend `ContactDto`.
+ * `id` equals the OxiCloud user UUID for contacts from the system address book.
+ * @typedef {Object} ContactItem
+ * @property {string}          id
+ * @property {string}          address_book_id
+ * @property {string}          uid             - vCard UID
+ * @property {string|null}     [full_name]
+ * @property {string|null}     [first_name]
+ * @property {string|null}     [last_name]
+ * @property {string|null}     [nickname]
+ * @property {ContactEmail[]}  email
+ * @property {string|null}     [organization]
+ * @property {string|null}     [title]
+ * @property {string|null}     [photo_url]
+ * @property {string}          created_at      - ISO-8601
+ * @property {string}          updated_at      - ISO-8601
+ * @property {string}          etag
+ */
+
+/**
+ * Mirrors the backend `AddressBookResponse`.
+ * @typedef {Object} AddressBookItem
+ * @property {string}       id
+ * @property {string}       name
+ * @property {string}       owner_id
+ * @property {string|null}  [description]
+ * @property {string|null}  [color]
+ * @property {boolean}      is_public
+ * @property {boolean}      is_readonly
+ * @property {boolean}      is_system
+ * @property {string}       created_at   - ISO-8601
+ * @property {string}       updated_at   - ISO-8601
+ */
+
diff --git a/static/js/features/ownerTooltip.js b/static/js/features/ownerTooltip.js
new file mode 100644
index 00000000..3349374d
--- /dev/null
+++ b/static/js/features/ownerTooltip.js
@@ -0,0 +1,121 @@
+// @ts-check
+
+/**
+ * Owner tooltip — shows "Shared by: <display name>" when hovering a
+ * `.file-item[data-owner-id]` element.
+ *
+ * Reuses the existing `#path-tooltip` DOM element (same position and style)
+ * so no extra CSS is needed.  The tooltip is hidden immediately on mouseleave
+ * and the display-name resolution is async-but-usually-instant because
+ * `systemUsers` is pre-fetched when the Shared-with-me section is entered.
+ *
+ * Usage:
+ *   ownerTooltip.init(containerEl)    — call after rendering items
+ *   ownerTooltip.destroy(containerEl) — call when leaving the section
+ */
+
+import { i18n } from '../core/i18n.js';
+import { systemUsers } from '../model/systemUsers.js';
+
+// ── Tooltip DOM ───────────────────────────────────────────────────────────────
+
+/** @returns {HTMLElement} */
+function _getOrCreateTooltip() {
+    let el = document.getElementById('path-tooltip');
+    if (!el) {
+        el = document.createElement('div');
+        el.id = 'path-tooltip';
+        el.className = 'path-tooltip hidden';
+        document.querySelector('.main-content')?.appendChild(el);
+    }
+    return el;
+}
+
+function _hide() {
+    document.getElementById('path-tooltip')?.classList.add('hidden');
+}
+
+// ── Event handlers ────────────────────────────────────────────────────────────
+
+/**
+ * @param {MouseEvent} e
+ */
+async function _onEnter(e) {
+    const item = /** @type {HTMLElement} */ (e.currentTarget);
+    const ownerId = item.dataset.ownerId;
+    if (!ownerId) return;
+
+    if (!systemUsers.isAvailable()) return;
+
+    const tooltip = _getOrCreateTooltip();
+
+    // Show immediately with a placeholder so the tooltip appears without lag.
+    const label = i18n.t('sharedwithme_sharedBy', 'Shared by');
+    tooltip.textContent = `${label}: …`;
+    tooltip.classList.remove('hidden');
+
+    // Resolve the name (usually instant from the pre-fetched cache).
+    const name = await systemUsers.getDisplayName(ownerId);
+
+    // Guard: don't update if the user already moved away.
+    if (!tooltip.classList.contains('hidden')) {
+        tooltip.textContent = `${label}: ${name}`;
+    }
+}
+
+function _onLeave() {
+    _hide();
+}
+
+// ── Listener registry (WeakMap for leak-free cleanup) ────────────────────────
+
+/**
+ * @typedef {{ enter: (e: MouseEvent) => void, leave: () => void }} Handlers
+ */
+
+/** @type {WeakMap<HTMLElement, Handlers>} */
+const _registry = new WeakMap();
+
+// ── Public API ────────────────────────────────────────────────────────────────
+
+/**
+ * Attach owner-tooltip listeners to every `.file-item[data-owner-id]`
+ * inside `container`.
+ * @param {HTMLElement} container
+ */
+function init(container) {
+    for (const item of container.querySelectorAll('.file-item[data-owner-id]')) {
+        const el = /** @type {HTMLElement} */ (item);
+        if (_registry.has(el)) continue; // already wired
+
+        /** @type {(e: MouseEvent) => void} */
+        const enter = (e) => {
+            _onEnter(e);
+        }; // intentionally discard the Promise
+        const leave = () => _onLeave();
+
+        el.addEventListener('mouseenter', enter);
+        el.addEventListener('mouseleave', leave);
+        _registry.set(el, { enter, leave });
+    }
+}
+
+/**
+ * Remove owner-tooltip listeners from all `.file-item` elements inside
+ * `container` and hide any visible tooltip.
+ * @param {HTMLElement} container
+ */
+function destroy(container) {
+    for (const item of container.querySelectorAll('.file-item')) {
+        const el = /** @type {HTMLElement} */ (item);
+        const h = _registry.get(el);
+        if (h) {
+            el.removeEventListener('mouseenter', h.enter);
+            el.removeEventListener('mouseleave', h.leave);
+            _registry.delete(el);
+        }
+    }
+    _hide();
+}
+
+export const ownerTooltip = { init, destroy };
diff --git a/static/js/model/addressBook.js b/static/js/model/addressBook.js
new file mode 100644
index 00000000..460201f1
--- /dev/null
+++ b/static/js/model/addressBook.js
@@ -0,0 +1,179 @@
+// @ts-check
+
+/**
+ * Address Book model.
+ *
+ * Provides access to all address books (user-owned + shared + the virtual
+ * system book) and their contacts.  Serves as the single source of truth for
+ * contact data across the application — sharing dialogs, owner tooltips, etc.
+ *
+ * Caching strategy
+ * ─────────────────
+ * • System book  — cached for the whole session (contacts = OxiCloud users,
+ *                  changes rarely and requires a page reload to pick up anyway).
+ * • User books   — cached on first load; call `invalidate(bookId)` after a
+ *                  write (create/update/delete contact) to force a re-fetch.
+ *
+ * The system book returns 404 when `OXICLOUD_EXPOSE_SYSTEM_USERS` is disabled.
+ * In that case `isSystemAvailable()` returns false and all callers degrade
+ * gracefully.
+ */
+
+/** @import {AddressBookItem, ContactItem} from '../core/types.js' */
+
+/** Sentinel id for the virtual system address book. */
+export const SYSTEM_BOOK_ID = 'system';
+
+/** @type {AddressBookItem[] | null} */
+let _books = null;
+
+/** @type {Map<string, ContactItem[]>} bookId → contacts (loaded books) */
+const _contactCache = new Map();
+
+/** @type {Map<string, Promise<ContactItem[]>>} bookId → in-flight request */
+const _inflight = new Map();
+
+/**
+ * `null`  = not yet attempted
+ * `true`  = loaded successfully at least once
+ * `false` = 404 / feature disabled
+ * @type {boolean | null}
+ */
+let _systemAvailable = null;
+
+// ── Address books ─────────────────────────────────────────────────────────────
+
+/**
+ * List all address books accessible to the current user.
+ * Result is cached for the session.
+ * @returns {Promise<AddressBookItem[]>}
+ */
+async function listBooks() {
+    if (_books !== null) return _books;
+    const res = await fetch('/api/address-books', { credentials: 'same-origin' });
+    if (!res.ok) throw new Error(`addressBook.listBooks: HTTP ${res.status}`);
+    _books = /** @type {AddressBookItem[]} */ (await res.json());
+    return _books;
+}
+
+// ── Contacts ──────────────────────────────────────────────────────────────────
+
+/**
+ * List contacts in an address book.
+ *
+ * Results are cached per book id.  For the system book, a 404 is treated as
+ * "feature disabled" — an empty array is returned and `isSystemAvailable()`
+ * will report false.
+ *
+ * @param {string} bookId
+ * @param {{ limit?: number, offset?: number }} [opts]
+ * @returns {Promise<ContactItem[]>}
+ */
+async function listContacts(bookId, opts = {}) {
+    if (_contactCache.has(bookId)) {
+        return /** @type {ContactItem[]} */ (_contactCache.get(bookId));
+    }
+
+    if (_inflight.has(bookId)) {
+        return /** @type {Promise<ContactItem[]>} */ (_inflight.get(bookId));
+    }
+
+    const p = (async () => {
+        try {
+            const params = new URLSearchParams();
+            if (opts.limit !== undefined) params.set('limit', String(opts.limit));
+            if (opts.offset !== undefined) params.set('offset', String(opts.offset));
+            const qs = params.size ? `?${params}` : '';
+
+            const res = await fetch(`/api/address-books/${encodeURIComponent(bookId)}/contacts${qs}`, {
+                credentials: 'same-origin',
+                cache: 'default'
+            });
+
+            if (res.status === 404 && bookId === SYSTEM_BOOK_ID) {
+                _systemAvailable = false;
+                _contactCache.set(bookId, []);
+                return /** @type {ContactItem[]} */ ([]);
+            }
+
+            if (!res.ok) {
+                throw new Error(`addressBook.listContacts(${bookId}): HTTP ${res.status}`);
+            }
+
+            const contacts = /** @type {ContactItem[]} */ (await res.json());
+            _contactCache.set(bookId, contacts);
+            if (bookId === SYSTEM_BOOK_ID) _systemAvailable = true;
+            return contacts;
+        } finally {
+            _inflight.delete(bookId);
+        }
+    })();
+
+    _inflight.set(bookId, p);
+    return p;
+}
+
+/**
+ * Invalidate the contact cache for a given book so the next `listContacts`
+ * call re-fetches from the server.  Call after any write operation.
+ * @param {string} bookId
+ */
+function invalidate(bookId) {
+    _contactCache.delete(bookId);
+}
+
+// ── Search ────────────────────────────────────────────────────────────────────
+
+/**
+ * Search contacts across one or more address books.
+ *
+ * Matching is case-insensitive against the full name, first+last name, and
+ * primary email.  Books are fetched and cached on first use.
+ *
+ * @param {string}   query
+ * @param {string[]} [bookIds]  - Books to search.  Defaults to all cached books.
+ *                                Pass `[SYSTEM_BOOK_ID]` to restrict to OxiCloud users.
+ * @returns {Promise<ContactItem[]>}
+ */
+async function searchContacts(query, bookIds) {
+    const ids = bookIds ?? [..._contactCache.keys()];
+    const q = query.toLowerCase().trim();
+    if (!q) return [];
+
+    /** @type {ContactItem[]} */
+    const results = [];
+
+    for (const id of ids) {
+        const contacts = await listContacts(id);
+        for (const c of contacts) {
+            const fullName = [c.first_name, c.last_name].filter(Boolean).join(' ') || c.full_name || '';
+            const primaryEmail = c.email?.find((e) => e.is_primary)?.email ?? c.email?.[0]?.email ?? '';
+
+            if (fullName.toLowerCase().includes(q) || primaryEmail.toLowerCase().includes(q)) {
+                results.push(c);
+            }
+        }
+    }
+
+    return results;
+}
+
+// ── Status ────────────────────────────────────────────────────────────────────
+
+/**
+ * Whether the system address book is (or may be) available.
+ * Returns `true` when status is unknown (not yet fetched).
+ * Returns `false` only after a confirmed 404.
+ * @returns {boolean}
+ */
+function isSystemAvailable() {
+    return _systemAvailable !== false;
+}
+
+export const addressBook = {
+    listBooks,
+    listContacts,
+    invalidate,
+    searchContacts,
+    isSystemAvailable
+};
diff --git a/static/js/model/systemUsers.js b/static/js/model/systemUsers.js
new file mode 100644
index 00000000..fb035ccf
--- /dev/null
+++ b/static/js/model/systemUsers.js
@@ -0,0 +1,80 @@
+// @ts-check
+
+/**
+ * System-users convenience layer.
+ *
+ * Thin wrapper over `addressBook.listContacts(SYSTEM_BOOK_ID)` that
+ * provides a userId → display-name index.  Used wherever a grant's
+ * `granted_by` UUID needs to be shown as a human-readable name
+ * (owner tooltips, share dialogs, etc.).
+ *
+ * Falls back gracefully when the system address book is disabled
+ * server-side (`OXICLOUD_EXPOSE_SYSTEM_USERS` not set): `isAvailable()`
+ * returns false and `getDisplayName()` returns a shortened UUID.
+ */
+
+/** @import {ContactItem} from '../core/types.js' */
+
+import { addressBook, SYSTEM_BOOK_ID } from './addressBook.js';
+
+/** @type {Map<string, string> | null} userId → display name, built lazily */
+let _index = null;
+
+/**
+ * Derive the best human-readable name from a contact.
+ * Priority: "First Last" → full_name → primary email → shortened id.
+ * @param {ContactItem} c
+ * @returns {string}
+ */
+function _nameFor(c) {
+    const parts = /** @type {string[]} */ ([c.first_name, c.last_name].filter(Boolean));
+    if (parts.length) return parts.join(' ');
+    if (c.full_name) return c.full_name;
+    const mail = c.email?.find((e) => e.is_primary)?.email ?? c.email?.[0]?.email;
+    if (mail) return mail;
+    return `${c.id.slice(0, 8)}…`;
+}
+
+/**
+ * Ensure the index is built (idempotent).
+ * @returns {Promise<void>}
+ */
+async function _ensureIndex() {
+    if (_index !== null) return;
+    const contacts = await addressBook.listContacts(SYSTEM_BOOK_ID);
+    _index = new Map(contacts.map((c) => [c.id, _nameFor(c)]));
+}
+
+// ── Public API ────────────────────────────────────────────────────────────────
+
+/**
+ * Start loading the system address book in the background.
+ * Safe to call multiple times — subsequent calls are no-ops once loaded.
+ */
+function prefetch() {
+    if (!addressBook.isSystemAvailable()) return;
+    _ensureIndex(); // intentionally fire-and-forget
+}
+
+/**
+ * Resolve a user UUID to a display name.
+ * Awaits the first load if not yet cached; subsequent calls resolve instantly.
+ *
+ * @param {string} userId
+ * @returns {Promise<string>}
+ */
+async function getDisplayName(userId) {
+    await _ensureIndex();
+    return _index?.get(userId) ?? `${userId.slice(0, 8)}…`;
+}
+
+/**
+ * Returns `false` only after a confirmed 404 from the server (feature
+ * disabled).  Returns `true` when status is unknown or the book loaded OK.
+ * @returns {boolean}
+ */
+function isAvailable() {
+    return addressBook.isSystemAvailable();
+}
+
+export const systemUsers = { prefetch, getDisplayName, isAvailable };
diff --git a/static/js/views/sharedWithMe/sharedWithMeView.js b/static/js/views/sharedWithMe/sharedWithMeView.js
index 24ff8171..bcde3ac8 100644
--- a/static/js/views/sharedWithMe/sharedWithMeView.js
+++ b/static/js/views/sharedWithMe/sharedWithMeView.js
@@ -16,8 +16,8 @@
 
 import { ui } from '../../app/ui.js';
 import { i18n } from '../../core/i18n.js';
-import { ownerTooltip } from '../../features/ownerTooltip.js';
 import { multiSelect } from '../../features/files/multiSelect.js';
+import { ownerTooltip } from '../../features/ownerTooltip.js';
 import { grants } from '../../model/grants.js';
 import { systemUsers } from '../../model/systemUsers.js';
 
@@ -201,9 +201,7 @@ const sharedWithMeView = {
      */
     _stampOwnerIds(container, ownerMap) {
         for (const [itemId, ownerId] of ownerMap) {
-            const el = container.querySelector(
-                `[data-folder-id="${itemId}"], [data-file-id="${itemId}"]`
-            );
+            const el = container.querySelector(`[data-folder-id="${itemId}"], [data-file-id="${itemId}"]`);
             if (el instanceof HTMLElement) {
                 el.dataset.ownerId = ownerId;
             }