How many books by William Shakespeare were published by Penguin Classics?
"William Shakespeare" is the author_name; "Penguin Classics" is the publisher_name
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 INNER JOIN publisher AS T4 ON T4.publisher_id = T1.publisher_id WHERE T3.author_name = 'William Shakespeare' AND T4.publisher_name = 'Penguin Classics'