I don't think there is any book for step 2 that is as good as First Aid was for step 1. I read one time through the Master the Boards book, and did not feel like it helped very much. For the most part, I pretty much only used uworld, wrote down the explanations for the questions I got wrong, and reviewed my wrong questions every night, and did really well on step 2. I also took a practice nbme every weekend to help gauge my progress.