最も異なる本を数えるのに援助が必要

これを試してください：

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