public static void PrintBooksByAuthorAndTotalPrice(Library library)
        {

			// group books in library by author, then order them descending, by their total price (using .Sum())
			// and finaly order them by author’s name lexicographically
            foreach (var book in library.Books
                .GroupBy(b => b.Autor)
                .OrderByDescending(x => x.Sum(p => p.Price))
                .ThenBy(k => k.Key))
            {
                Console.WriteLine($"{book.Key} -> {book.Sum(p => p.Price):f2}");
            }
        }