How do I define this relationship with code-first? - asp.net-web-api

Question

I'm trying to create a library of books.
I separated data and users context/database for security reasons (and clarity) but is it that useful?
Specially since BookOfUserEntity.Id should be really a composite key between UserId & BookId.
Something like :
public class BookOfUserEntity
{
[Key]
public Guid UserId { get; set; }
[Key]
public int BookId { get; set; }
}
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<BookOfUserEntity>()
.HasKey(b => new { b.UserId, b.BookId });
}
But then if I create a composite key what should be HistoryEntity.BookOfUserEntityId type & value?
I also separated BookEntity & BookOfUserEntity to avoid duplicated data and reuse what can be reused (MainCoverArtId etc...), but this added complexity - should I go back to a simpler model?
public class BookEntity
{
public int Id { get; set; }
public Guid? MainCoverArtId { get; set; }
...
public virtual List<BookOfUserEntity>? BookOfUserEntities { get; set; } // Only purpose is to know how many Users have this very book in their list.
}
public class BookOfUserEntity // Bad name here don't mention it
{
public int Id { get; set; }
public Guid UserId { get; set; }
public int BookId { get; set; }
public BookEntity Book { get; set; } // Useful? Should be virtual?
public List<HistoryEntity>? HistoryEntities { get; set; }
...
}
public class HistoryEntity
{
public int Id { get; set; }
public int BookOfUserEntityId { get; set; }
public BookOfUserEntity BookOfUserEntity { get; set; } // Same questions
public int Vol { get; set; }
public double Chapter { get; set; }
public DateTime? ReadDate { get; init; }
}