Write SQL query to solve given problem: List all the authors who wrote fewer pages than the average.. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T3.author_name FROM book AS T1 INNER JOIN book_author AS T2 ON T1.book_id = T2.book_id INNER JOIN author AS T3 ON T3.author_id = T2.author_id WHERE T1.num_pages < ( SELECT AVG(num_pages) FROM book )