---

UNION ALL
SELECT YEAR(date_step_end) AS Год,
MONTHNAME(date_step_end) AS Месяц,
SUM(bb.amount*price) AS Сумма
FROM buy_book bb
JOIN buy_step bs ON bb.buy_id = bs.buy_id
AND bs.date_step_end
AND bs.step_id = 1
JOIN book USING(book_id)
GROUP BY Год,Месяц
ORDER BY Месяц, Год;