//IEnumerable<Book>.Select 將序列中的Authors元素投影到新表中.
            IEnumerable<List<Author>> EnumerableOfListOfAuthors = Books.Select(book => book.Authors);

            foreach (var listOfAuthors in EnumerableOfListOfAuthors)
            {
                foreach (Author auth in listOfAuthors)
                {
                    Output.Items.Add(auth.Name); //新增到ListBox裡面
                }
            }

            //IEnumerable<Book>.SelectMany  將序列的每個元素投影到 IEnumerable<T> 並將結果序列合併為一個序列。
            IEnumerable<Author> authors = Books.SelectMany(book => book.Authors);
            foreach (Author auth in authors)
            {
                Output2.Items.Add(auth.Name);
            }