Tente isto:
SELECT c.cust_id, COUNT(DISTINCT od.book_id) bookCnt
FROM a_bkorders.customers c
INNER JOIN a_bkorders.order_headers oh ON c.cust_id = oh.cust_id
INNER JOIN a_bkorders.order_details od ON oh.order_id = od.order_id
GROUP BY c.cust_id
ORDER BY bookCnt DESC
