using System;

using System.Collections.Generic;

using System.Linq;

​

namespace Small

{

    public class Program

    {

        public static void Main(string[] args)

        {

            var Books = new List<Book>

            {

                new Book { Id = 12, Title = "Title 1" },

                new Book { Id = 13, Title = "Title 2" },

                new Book { Id = 14, Title = "Title 3" }

            };

​

            var Authors = new List<Author>

            {

                new Author {Id = 1, BookId =   12 , Name = "Steeve"},

                new Author {Id = 2, BookId =   13 , Name = "Michel" },

                new Author {Id = 3, BookId =   13 , Name = "Robert" }

            };

​

            var query = from book in Books

                        join author in Authors on book.Id equals author.BookId into gr1

                        select new { BookId = book.Id, AuthorId = gr1.LastOrDefault() == null ? "Null" : gr1.LastOrDefault().Id.ToString() };

​

            var result = query.ToList();

            foreach (var thisResult in result)

            {

                Console.WriteLine("{0}\t{1}", thisResult.BookId, thisResult.AuthorId);

            }

        }

    }

    class Author

    {

        public int BookId { get; set; }

        public int Id { get; set; }

        public string Name { get; set; }

    }

    class Book

    {

        public int Id { get; set; }

        public string Title { get; set; }

    }

}

​