public Library()
        {
            IComparer <Book> comparer = new BookComparer();

            this.books = new SortedSet <Book>(comparer);
        }
        public Library(params Book[] booksArray) // IEnumerable<Book> booksArray
        {
            IComparer <Book> comparer = new BookComparer();

            this.books = new SortedSet <Book>(booksArray, comparer);
        }