diff --git a/internal/audiobooks/abs/collections_handler.go b/internal/audiobooks/abs/collections_handler.go
index 94242f2d..ac25397c 100644
--- a/internal/audiobooks/abs/collections_handler.go
+++ b/internal/audiobooks/abs/collections_handler.go
@@ -223,6 +223,103 @@ func (h *Handler) handleUpdateCollection(w http.ResponseWriter, r *http.Request)
 	writeJSON(w, http.StatusOK, h.collectionFullShape(r, persisted))
 }
 
+// handleAddCollectionBook — POST /collections/{id}/book/{bookId}.
+// Owner-gated. Validates the item exists via MediaStore (returns 404
+// for unknown items). Idempotent: re-adding is a silent no-op.
+// Returns the parent collection's full-shape with updated books[].
+func (h *Handler) handleAddCollectionBook(w http.ResponseWriter, r *http.Request) {
+	a, ok := absAuthFrom(r)
+	if !ok || a.UserID == "" {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+	if h.deps.CollectionStore == nil {
+		http.Error(w, "collection not found", http.StatusNotFound)
+		return
+	}
+	id := chiURLID(r)
+	bookID := chi.URLParam(r, "bookId")
+	if bookID == "" {
+		http.Error(w, "bookId required", http.StatusBadRequest)
+		return
+	}
+
+	c, err := h.deps.CollectionStore.GetCollection(r.Context(), id)
+	if errors.Is(err, ErrNotFound) || (err == nil && c.UserID != a.UserID) {
+		http.Error(w, "collection not found", http.StatusNotFound)
+		return
+	}
+	if err != nil {
+		slog.Error("abs collection get-for-add failed", "err", err, "id", id)
+		http.Error(w, "collection get failed", http.StatusInternalServerError)
+		return
+	}
+
+	// Item validation — avoid orphan refs.
+	item, err := h.deps.MediaStore.GetAudiobookByID(r.Context(), bookID)
+	if err != nil || item == nil {
+		http.Error(w, "item not found", http.StatusNotFound)
+		return
+	}
+
+	if err := h.deps.CollectionStore.AddCollectionItem(r.Context(), id, bookID); err != nil {
+		slog.Error("abs collection add-item failed", "err", err, "id", id, "book", bookID)
+		http.Error(w, "collection persist failed", http.StatusInternalServerError)
+		return
+	}
+
+	// Re-fetch to surface updated_at bump.
+	persisted, err := h.deps.CollectionStore.GetCollection(r.Context(), id)
+	if err != nil {
+		persisted = c
+	}
+	writeJSON(w, http.StatusOK, h.collectionFullShape(r, persisted))
+}
+
+// handleRemoveCollectionBook — DELETE /collections/{id}/book/{bookId}.
+// Owner-gated. Idempotent: removing a non-member is a no-op.
+// Returns the parent collection's full-shape with updated books[].
+func (h *Handler) handleRemoveCollectionBook(w http.ResponseWriter, r *http.Request) {
+	a, ok := absAuthFrom(r)
+	if !ok || a.UserID == "" {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+	if h.deps.CollectionStore == nil {
+		http.Error(w, "collection not found", http.StatusNotFound)
+		return
+	}
+	id := chiURLID(r)
+	bookID := chi.URLParam(r, "bookId")
+	if bookID == "" {
+		http.Error(w, "bookId required", http.StatusBadRequest)
+		return
+	}
+
+	c, err := h.deps.CollectionStore.GetCollection(r.Context(), id)
+	if errors.Is(err, ErrNotFound) || (err == nil && c.UserID != a.UserID) {
+		http.Error(w, "collection not found", http.StatusNotFound)
+		return
+	}
+	if err != nil {
+		slog.Error("abs collection get-for-remove failed", "err", err, "id", id)
+		http.Error(w, "collection get failed", http.StatusInternalServerError)
+		return
+	}
+
+	if err := h.deps.CollectionStore.RemoveCollectionItem(r.Context(), id, bookID); err != nil {
+		slog.Error("abs collection remove-item failed", "err", err, "id", id, "book", bookID)
+		http.Error(w, "collection delete failed", http.StatusInternalServerError)
+		return
+	}
+
+	persisted, err := h.deps.CollectionStore.GetCollection(r.Context(), id)
+	if err != nil {
+		persisted = c
+	}
+	writeJSON(w, http.StatusOK, h.collectionFullShape(r, persisted))
+}
+
 // handleDeleteCollection — DELETE /collections/{id}.
 // Owner-only. Cascade drops abs_collection_items via FK CASCADE.
 // 204 on success; 404 for unknown or non-owned.
diff --git a/internal/audiobooks/abs/collections_handler_test.go b/internal/audiobooks/abs/collections_handler_test.go
index 88746470..09b4a2c6 100644
--- a/internal/audiobooks/abs/collections_handler_test.go
+++ b/internal/audiobooks/abs/collections_handler_test.go
@@ -460,3 +460,104 @@ func TestCollection_Delete_NonOwner_404(t *testing.T) {
 		t.Errorf("collection wrongly deleted: %v", err)
 	}
 }
+
+func TestCollection_AddBook_Owner_HydratesInResponse(t *testing.T) {
+	hb := newCollectionsHarness(t, "book-1")
+	id := createCollectionForUser(t, hb, "1", "", `{"name":"x"}`)
+
+	rec := dispatchABSWithParams(http.MethodPost, "/api/collections/"+id+"/book/book-1",
+		map[string]string{"id": id, "bookId": "book-1"}, nil, "1", "", hb.H.handleAddCollectionBook)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200; body=%s", rec.Code, rec.Body.String())
+	}
+	var got map[string]any
+	_ = json.Unmarshal(rec.Body.Bytes(), &got)
+	books, _ := got["books"].([]any)
+	if len(books) != 1 {
+		t.Fatalf("books len = %d, want 1", len(books))
+	}
+	entry := books[0].(map[string]any)
+	if entry["id"] != "book-1" {
+		t.Errorf("book entry id = %v, want book-1", entry["id"])
+	}
+	if _, has := entry["media"]; !has {
+		t.Errorf("book entry missing media hydration: %v", entry)
+	}
+}
+
+func TestCollection_AddBook_Idempotent(t *testing.T) {
+	hb := newCollectionsHarness(t, "book-1")
+	id := createCollectionForUser(t, hb, "1", "", `{"name":"x"}`)
+
+	_ = dispatchABSWithParams(http.MethodPost, "/api/collections/"+id+"/book/book-1",
+		map[string]string{"id": id, "bookId": "book-1"}, nil, "1", "", hb.H.handleAddCollectionBook)
+	rec := dispatchABSWithParams(http.MethodPost, "/api/collections/"+id+"/book/book-1",
+		map[string]string{"id": id, "bookId": "book-1"}, nil, "1", "", hb.H.handleAddCollectionBook)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("second add status = %d, want 200 (idempotent); body=%s", rec.Code, rec.Body.String())
+	}
+	var got map[string]any
+	_ = json.Unmarshal(rec.Body.Bytes(), &got)
+	books, _ := got["books"].([]any)
+	if len(books) != 1 {
+		t.Errorf("books len after double-add = %d, want 1", len(books))
+	}
+}
+
+func TestCollection_AddBook_UnknownItem_404(t *testing.T) {
+	hb := newCollectionsHarness(t /* no known items */)
+	id := createCollectionForUser(t, hb, "1", "", `{"name":"x"}`)
+
+	rec := dispatchABSWithParams(http.MethodPost, "/api/collections/"+id+"/book/ghost",
+		map[string]string{"id": id, "bookId": "ghost"}, nil, "1", "", hb.H.handleAddCollectionBook)
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("status = %d, want 404 (item not found); body=%s", rec.Code, rec.Body.String())
+	}
+}
+
+func TestCollection_AddBook_NonOwner_404(t *testing.T) {
+	hb := newCollectionsHarness(t, "book-1")
+	id := createCollectionForUser(t, hb, "1", "", `{"name":"mine"}`)
+
+	rec := dispatchABSWithParams(http.MethodPost, "/api/collections/"+id+"/book/book-1",
+		map[string]string{"id": id, "bookId": "book-1"}, nil, "2", "", hb.H.handleAddCollectionBook)
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("status = %d, want 404 (no leak); body=%s", rec.Code, rec.Body.String())
+	}
+}
+
+func TestCollection_RemoveBook_Idempotent(t *testing.T) {
+	hb := newCollectionsHarness(t, "book-1")
+	id := createCollectionForUser(t, hb, "1", "", `{"name":"x"}`)
+
+	// Remove book that was never added — should be 200 with empty books.
+	rec := dispatchABSWithParams(http.MethodDelete, "/api/collections/"+id+"/book/book-1",
+		map[string]string{"id": id, "bookId": "book-1"}, nil, "1", "", hb.H.handleRemoveCollectionBook)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200; body=%s", rec.Code, rec.Body.String())
+	}
+	var got map[string]any
+	_ = json.Unmarshal(rec.Body.Bytes(), &got)
+	books, _ := got["books"].([]any)
+	if len(books) != 0 {
+		t.Errorf("books len = %d, want 0", len(books))
+	}
+}
+
+func TestCollection_RemoveBook_NonOwner_404(t *testing.T) {
+	hb := newCollectionsHarness(t, "book-1")
+	id := createCollectionForUser(t, hb, "1", "", `{"name":"mine"}`)
+	_ = dispatchABSWithParams(http.MethodPost, "/api/collections/"+id+"/book/book-1",
+		map[string]string{"id": id, "bookId": "book-1"}, nil, "1", "", hb.H.handleAddCollectionBook)
+
+	rec := dispatchABSWithParams(http.MethodDelete, "/api/collections/"+id+"/book/book-1",
+		map[string]string{"id": id, "bookId": "book-1"}, nil, "2", "", hb.H.handleRemoveCollectionBook)
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("status = %d, want 404; body=%s", rec.Code, rec.Body.String())
+	}
+	// User 1's items must be intact.
+	items, _ := hb.Coll.ListCollectionItems(context.Background(), id)
+	if len(items) != 1 {
+		t.Errorf("items len = %d, want 1 (non-owner remove leaked)", len(items))
+	}
+}