W ILibrary:
[OperationContract]
        List<Book> ListOfBorrowedBooks();


W Library:
 public List<Book> ListOfBorrowedBooks()
        {
            List<Book> borrowedBooks = new List<Book>();

            foreach (Book b in books)
            {
                if (b.Status.BorrowedStatus)
                {
                    borrowedBooks.Add(b);
                }
            }
            return borrowedBooks;
        }

U clienta:
LibraryClient library = new LibraryClient();

  private static void ListOfBorrowedBooks()
        {
            Console.WriteLine("List of borrowed books");
            books = library.ListOfBorrowedBooks();
            PrintBorrowedBooks();
        }