@Test
    public void test() {
        getBooks()
                .flatMapSingle(bookList -> Flowable.fromIterable(bookList)
                        .flatMap(book -> getCategories(book.id)
                                .map(categories -> {
                                    book.categories = categories;
                                    return book;
                                }))
                        .flatMap(book -> getAuthors(book.id)
                                .map(authors -> {
                                    book.authors = authors;
                                    return book;
                                }))
                        .toList())
                .subscribe(bookList -> {
                    //done
                });
    }

    Flowable<List<Book>> getBooks() {
        return Flowable.just(Arrays.asList(new Book(), new Book()));
    }

    Flowable<List<Category>> getCategories(int id) {
        return Flowable.just(Arrays.asList(new Category(), new Category()));
    }

    Flowable<List<Author>> getAuthors(int id) {
        return Flowable.just(Arrays.asList(new Author(), new Author()));
    }

    static class Book {
        int id = 0;
        public List<Author> authors;
        public List<Category> categories;
    }

    static class Author {}

    static class Category {}