SELECT
  g.id AS genre_id,
  g.name AS genre_name,
  t2.books_in_genre,
  b.id AS book_id,
  b.name AS book_name,
  b.date_added
FROM book b
RIGHT JOIN book_genre bg ON b.id = bg.book_id
RIGHT JOIN genre g ON g.id = bg.genre_id
JOIN (
    SELECT g.id genre_id, COUNT(b.id) books_in_genre, MAX(b.date_added) newest_book
    FROM book b
    RIGHT JOIN book_genre bg ON b.id = bg.book_id
    RIGHT JOIN genre g ON g.id = bg.genre_id
    GROUP BY g.id
) t2
ON g.id = t2.genre_id
AND (b.date_added = t2.newest_book OR b.date_added IS NULL AND t2.newest_book IS NULL)
ORDER BY t2.books_in_genre DESC