Entity framework many to many problem

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C#

.NET

VS2012

entity-framework

hi,
i have problem on adding new entities to my context in entity framework code first
problem is that when i want to add new author to book or book to author is says :
"Object reference not set to an instance of an object." because author of book or book of author is null... while i`v set that.
here is my code:

class Program
    {
        static void Main(string[] args)
        {
            var db = new Source();
            Publisher publisher = db.Publisher.Single( p=> p.Title == "Press");
            Book book = new Book() { ISBN = "123424", Publisher = publisher, Title = "Intro to .NET" };
            book.Authors.Add(new Author() { FirstName = "Bill", LastName = "Gates"});

            db.Books.Add(book);
            db.SaveChanges();
        }
    }

    public class Source: DbContext
    {
        public Source(): base("Main")
        {

        }

        public DbSet<Publisher> Publisher { get; set; }
        public DbSet<Author> Authors { get; set; }
        public DbSet<Book> Books { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Publisher>().HasKey(p => p.ID);
            modelBuilder.Entity<Book>().Property(p => p.Title).HasMaxLength(200);
        }
    }

    public class Publisher
    {
        public long ID { get; set; }

        public string Title { get; set; }
        public string Email { get; set; }
        public string Address { get; set; }
    }

    public class Book
    {
        public long ID { get; set; }

        public string Title { get; set; }
        public string ISBN { get; set; }

        public List<Author> Authors { get; set; }
        public Publisher Publisher { get; set; }
    }

    public class Author
    {
        public long ID { get; set; }

        public string FirstName { get; set; }
        public string LastName { get; set; }

        public List<Book> Books { get; set; }
    }