Ten Actual LSAT's book is not a waste. It contains exactly what it says it contains, 10 LSATs (numbers 7-18, very old). That said, you should probably also get Ten More, Next Ten, and the most recent tests (order individually from LSAC) if you plan on a really thorough prep. If you don't want to spend that much money, I got by on Ten Actual and Next Ten along with the 6 or so most recent tests.
People say that the Ten Actual book is useless because the test has changed considerably since these early modern admissions. Post 2003 or so the logic games are generally easier and the rc is somewhat harder. The last few admissions have a comparative reading passage in the rc section. Some of the older game types are quite wild and haven't shown up lately.
Prepping from Ten Actual is fine. I think its good because the more recent tests are, to me at least, logically easier (esp. games). The scales, however, are tougher-- mitigating this difference. The comparative difficulty of the more recent test is a point of contention on this board, though. That they are easier is my opinion.
It is essential that you take some of the most recent (post 2003 or so) tests before the real thing.
For an example of how the newest tests and the Ten Actual tests feel a bit different, check out the June 2007 test. It is free on LSAC's website. That admission is the best example of a logically relatively easy test with a wicked scale (-8 for 170 I think). Compare this to say test no. 7 in Ten Actual where -15 earned a 170.
Best of luck in your prep.