If you didn't know, then you didn't deserve the job.
Can you see a column that both tables have in column ? It's BID. So the sql is
select b.bname, a.aname from book b inner join author a on b.bid = a.bid
to get a list of all authors and books. However, what you want, is books having more than one author. So the answer is:
select a.aname, count(b.bname) from book b inner join author a on b.bid = a.bid group by a.aname having count(b.bname) > 1