BooksCount.cs

static void Main(string[] args)
{
	Mapper.Initialize(cfg =>
	{
		cfg.CreateMap<Library, LibraryDto>();
	});

	using(var context = new TestContext())
	{
		context.Libraries.ProjectTo<LibraryDto>().Dump();
	}
}

public class DatabaseInitializer : DropCreateDatabaseAlways<TestContext>
{
  protected override void Seed(TestContext context)
  {
  	context.Libraries.AddRange(new[]{new Library { Books = {new Book {}, new Book {}}}});
  }
}

public class TestContext : DbContext
{
  public TestContext()
  {
      //Database.Log = s=>Debug.WriteLine(s);
      Database.SetInitializer<TestContext>(new DatabaseInitializer());
  }
  public DbSet<Library> Libraries { get; set; }
}

public class Library
{
	public int Id { get; set; }
	public List<Book> Books { get; set; }

	public Library()
	{
		Books = new List<Book>();
	}
}

public class Book
{
	public int Id { get; set; }
}

public class LibraryDto
{
	public int Id { get; set; }
	public int BooksCount { get; set; }
}