What is the average number of book pages written by Zilpha Keatley Snyder?
"Zilpha Keatley Snyder" is the author_name; average number of book pages refers to AVG(num_pages)
SELECT AVG(T3.num_pages) FROM book_author AS T1 INNER JOIN author AS T2 ON T1.author_id = T2.author_id INNER JOIN book AS T3 ON T3.book_id = T1.book_id WHERE T2.author_name = 'Zilpha Keatley Snyder'