As a supplement to any test prep book you get I think purchasing LSAC's 10 actual preptests (and then the 10 NEW ones and the ones with comparative reading) is invaluable to true test preparation.
Since they are the closest thing to the actual test you can get your hands on, you can go through them as if you're taking the real test 10 times. The only problem is they do not provide reasons for the right answers, but most of the time I am able to work out where I went wrong and if I cannot I can ask for help (like I did in another topic in this forum, though people seem reluctant to respond).
I noticed the same thing when I was studying for the LSAT. Many of the practice tests would give you the correct answers, but wouldn't actually tell you why that particular conclusion was correct. Very frustrating for questions that are a bit more ambiguous or could logically have multiple correct answers.