Your first two purchases should be Kaplan's (or Princeton's, if you prefer) most current LSAT guide and 10 More Actual, Official LSAT PrepTests. Kaplan's book will give you a good intro to the test, along with 3 full tests with a detailed analysis for each question. The PrepTest book will give you plenty of real, recently-administered LSATs to take, though without question analysis. Just getting used to taking the test is the most important part of the preparation process, and between the two books, 13 full tests should be plenty.
If, after 4 or 5 tests, you're noticing your scores plateau around 165, and you want to bump them up, get Kaplan's LSAT 180. It is full of the toughest questions that Kaplan's crew could come up with (along with a few nasty ones you might see adapted from real questions on LSAC's PrepTests), and strategies on how to approach them. The regular, yearly guides are a great way to start studying, but they will only take you so far. LSAT 180 can take you the rest of the way, as its name implies. You should be warned beforehand though--these are THE TOUGHEST questions you will find anywhere. If you can handle these, most of the actual test will seem like cake.
If you burn through all of Kaplan's Tests--including the free online one--and all 10 Actual tests (Like I did), you can always get the older 10 Actual, Official LSAT PrepTests. It's still good prep; the only difference is that the tests are older--December '92 is the oldest; September '95 is the most recent. Some minor things have changed: the older tests are a little harder, which means a lower raw score here translates into a higher scaled score; the wording of questions is different; and some of the types of logic games that are on the newer tests aren't on these. However, if you've taken all the tests in the more recent book, and you know what to expect to see on the actual test, taking the PrepTests in this book is a good way to keep your practice going.
Also, both LSAC.org and Amazon have individual PrepTests available for $8 apiece. You can get all the most recent tests--sometimes right up to the one most recently administered. These aren't as good a deal as the books of 10, but seeing the most recent material--even if it's just 1 or 2 tests--is worth it. Though I haven't taken it, I've heard that PrepTest 31 (June 2000) has the most difficult Logic Game ever on it... that might be worth checking out.
Anyway, that's about it. If you want to prepare for the LSAT as thoroughly and effectively as possible, this is the way to do it. As for REA's book, don't bother.
