Who ordered the book with the cheapest price?
book with cheapest price refers to Min(price); who order means name of customer which refers to first_name, last_name
SELECT T3.first_name, T3.last_name FROM order_line AS T1 INNER JOIN cust_order AS T2 ON T2.order_id = T1.order_id INNER JOIN customer AS T3 ON T3.customer_id = T2.customer_id ORDER BY T1.price ASC LIMIT 1