How many books under 300 pages has HarperCollins Publishers published?
under 300 pages refers to num_pages < 300; 'HarperCollins Publishers" is the publisher_name
SELECT COUNT(*) FROM book AS T1 INNER JOIN publisher AS T2 ON T1.publisher_id = T2.publisher_id WHERE T2.publisher_name = 'HarperCollins Publishers' AND T1.num_pages < 300