I think it is not the matter of buying 1 book or 10 books, it is a matter of understanding your weaknesses and working on them.
has a good quality CD with a lot of tough questions. The only disadvantage of Kaplan
is that the scores do not reflect the real one's. It could be sometimes fun to practice and master the toughest questions, than when you will see the moderate and easy questions, you will find them very easy...