Well, you can fix it by using except on a query that gives all the book_id's where the author isn't one of 'Smith' or 'Jones'.

select book_id from authors
where
  author_name in ('Smith','Jones')
group by book_id
having count(*)=2
except
select book_id from authors
where
  author_name not in ('Smith','Jones');
[download]

You could also alter the where clause to make certain book_id doesn't have a different author:

select book_id from authors
where
  author_name in ('Smith','Jones')
  and book_id not in (
    select book_id from authors
    where
       author_name not in ('Smith','Jones')
  )
group by book_id
having count(*)=2
[download]

By the way, if anyone wants to read some of Joe Celko's old articles from DBMS magazine, here are some that are available from their website (the puzzles are really fun).