What is the name of the publisher that published the most books?
name of publisher refers to publisher_name; publisher published the most number of books refers to Max(Count(book_id))
SELECT T2.publisher_name FROM book AS T1 INNER JOIN publisher AS T2 ON T1.publisher_id = T2.publisher_id GROUP BY T2.publisher_name ORDER BY COUNT(T2.publisher_id) DESC LIMIT 1