Who is the author of the book with the biggest page count?
author refers to author_name, biggest page count refers to Max(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 ORDER BY T1.num_pages DESC LIMIT 1