public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
    public Author Author { get; set; }
    public ICollection<BookCategory> BookCategories { get; set; }
}
public class Category
{
    public int CategoryId { get; set; }
    public string CategoryName { get; set; }
    public ICollection<BookCategory> BookCategories { get; set; }
}
public class BookCategory
{
    public int BookId { get; set; }
    public Book Book { get; set; }
    public int CategoryId { get; set; }
    public Category Category { get; set; }
    // NOT IN ALL THE EXAMPLES: public int ID {get; set;}
}

modelBuilder.Entity<BookCategory>()
        .HasKey(bc => new { bc.BookId, bc.CategoryId });
    modelBuilder.Entity<BookCategory>()
        .HasOne(bc => bc.Book)
        .WithMany(b => b.BookCategories)
        .HasForeignKey(bc => bc.BookId);
    modelBuilder.Entity<BookCategory>()
        .HasOne(bc => bc.Category)
        .WithMany(c => c.BookCategories)
        .HasForeignKey(bc => bc.CategoryId);