Create interfaces for your clases. Depend on the interfaces. Make it testable.

InterfaceBasedDevelopmentWithTest.cs

    public class Author
    {
        public string Name { get; set; }
    }

    public class Book
    {
        public string Title { get; set; }
    }

    public interface IAuthorRepository
    {
        IEnumerable<Author> GetAuthors();
    }

    public interface IBookRepository
    {
        IEnumerable<Book> GetBooks();
    }

    public class LibraryService
    {
        private readonly IBookRepository _bookRepository;
        private readonly IAuthorRepository _authorRepository;

        public LibraryService(IBookRepository bookRepository, IAuthorRepository authorRepository)
        {
            _bookRepository = bookRepository;
            _authorRepository = authorRepository;
        }

        public string GetAllBooksAndAuthors()
        {
            StringBuilder builder = new StringBuilder();

            _bookRepository.GetBooks()
                .Select(b => b.Title)
                .ToList()
                .ForEach(title => builder.AppendFormat("Book title: {0}{1}", title, Environment.NewLine));

            builder.AppendLine();
            _authorRepository.GetAuthors()
                .Select(a => a.Name)
                .ToList()
                .ForEach(authorName => builder.AppendFormat("Author name: {0}{1}", authorName, Environment.NewLine));

            return builder.ToString();
        }
    }

    [TestFixture]
    public class LibraryServiceTest
    {
        private IBookRepository _bookRepository;
        private IAuthorRepository _authorRepository;
        private LibraryService _testee;

        [SetUp]
        public void SetUp()
        {
            _bookRepository = A.Fake<IBookRepository>();
            _authorRepository = A.Fake<IAuthorRepository>();
            _testee = new LibraryService(_bookRepository, _authorRepository);
        }

        [Test]
        public void GetAllBOoksAndAuthors_ShouldReturnFormatedListOfBooksAndAuthors()
        {
            //arrange
            A.CallTo(() => _bookRepository.GetBooks()).Returns(new [] { new Book { Title = "Clean Code" }, new Book {Title = "The Art of Unit Testing"} });
            A.CallTo(() => _authorRepository.GetAuthors()).Returns(new[] { new Author { Name = "Hänsel" }, new Author { Name = "Gretel" }, });

            //act
            var result = _testee.GetAllBooksAndAuthors();

            //assert
            result.Should().Be("Book title: Clean Code\r\nBook title: The Art of Unit Testing\r\n\r\nAuthor name: Hänsel\r\nAuthor name: Gretel\r\n");
        }
    }