public void AddBook(Book book)
 {
     try
     {
         _Books.Add(book.Title, book);
     }
     catch (ModifyBookException e)
     {
         Console.WriteLine(e.Message);
     }
 }
 public void DeleteBook(Book b)
 {
     try
     {
         if (b.BorrowedBy != null)
             b.BorrowedBy.ReturnBook();
         if (_Books.ContainsValue(b))
             _Books.Remove(b.Title);
     }
     catch (DeleteBookException e)
     {
         Console.WriteLine(e.Message);
     }
 }