|Given the table authorbook(authid INT, bookid INT), what query finds the books who have authors with more than one book in the table?
Even one level of recursion can induce a mild trance. Escape the trance by taking the problem one step at a time. First write the query that finds the authors with multiple books. Then join an outer query to that on authorid, and have the outer query select bookid:
FROM authorbook a1
INNER JOIN (
FROM authorbook a2
GROUP BY authid
) AS a3 ON a1.authid=a3.authid;