What is the average number of pages in the books written by Jennifer Crusie?
"Jennifer Crusie" is the author_name; average number of pages refers to AVG(num_pages)
SELECT AVG(T1.num_pages) 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 T3.author_name = 'Jennifer Crusie'