//BookVariableInfo и Book связь 1:1
 public BookVariableInfo FindByBookId(Int32 id)
 {
     using (BookMarketDbContext db = new BookMarketDbContext())
     {
         var items = db.FindBookVariableInfoByBookId(id);
         var item = items.FirstOrDefault<BookVariableInfo>();
         return item;
     }
 }