DummyApi.swift

class DummyApi: Api {

    var userBooks = "Book 1", "Book 2", "Book 3"

    func getBooks() -> Observable<[Book]> {
        return userBooks
    }

    func deleteBook(_ book: Book) -> Single<Book> {
         ...wait 0.5s...
         userBooks.remove(book)
         ...userBooks emits onNext...
    }
}