I frankly wouldn't bother buying one, because you can usually pick up what you need in lecture and/or First Aid for Step 1. However, High Yield Biostats is certainly enough detail.
For those really into statistics, I HIGHLY recommend Multivariate Stats and any other book written by Tabachnik and Fidell. Two personable women who write in an interesting, and often humorous style, making it much easier and enjoyable to learn the subject.