The ratio of hardback books to paperback books on a bookshelf is $6: 21$. What fraction of the books on the bookshelf are hardback books? Give your answer in its simplest form.

Step 1 ：Given the ratio of hardback books to paperback books is 6:21, we can represent the fraction of hardback books as \(\frac{6}{6+21}\)

Step 2 ：Simplify the fraction: \(\frac{6}{27} = \frac{2}{9}\)

Step 3 ：\(\boxed{\frac{2}{9}}\) is the fraction of hardback books on the bookshelf in its simplest form