How many of the books authored by Al Gore have less than 400 pages?
"AI Gore" is the author_name; have less than 400 pages refers to num_pages < 400
SELECT COUNT(*) 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 = 'Al Gore' AND T1.num_pages < 400