What is the average number of pages of David Coward's books?
number of pages refers to num_pages; average = Divide (Sum(num_pages), Count(book_id))
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 = 'David Coward'