create-reducer.ts

export const initialState: State = adapter.getInitialState({
  selectedBookId: null,
});

export const reducer = createReducer(
  initialState,
  on(
    BooksApiActions.searchSuccess,
    CollectionApiActions.loadBooksSuccess,
    (state, { books }) => adapter.addMany(books, state)
  ),
  on(BookActions.loadBook, (state, { book }) => adapter.addOne(book, state)),
  on(ViewBookPageActions.selectBook, (state, { id }) => ({
    ...state,
    selectedBookId: id,
  }))
);