List all the authors who wrote fewer pages than the average.
author refers to author_name; who wrote fewer pages than the average refers to num_pages < AVG(num_pages)
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 )