List books

Listing books is a different intent from saving one. It gets its own class. A BooksService with save and list methods would hide two intents behind one type and would grow a third method the moment you can place a book.

Create lib/src/application/book/list_books_use_case.dart:

import '../../domain/book/book_repository.dart';
import '../../domain/book/entities/book.dart';

class ListBooksUseCase {
  const ListBooksUseCase(this._books);

  final BookRepository _books;

  Future<List<Book>> execute() => _books.list();
}

The body is one call. That is the point. The use case is still the type the endpoint calls. The endpoint does not receive a BookRepository. If listing later needs a filter, the change is a command object on this class, not a new query inside the endpoint.

Next: tests with a fake repository.