Honestly I wouldn't buy any books for the ACT (or the SAT for that matter). There is no real way to prepare for these tests, only through schooling can you be prepared. Just get a good nights rest and have confidence when taking it. Thats all you need.
The only thing books will do is give you an idea of what the questions are like. There's no possible way to prepare for every single question they could possibly ask, so the books just kind of give you a formula for how they're phrased and constructed and tips to do better.
Getting a book honestly can't hurt. I took the ACTs twice, the first time I got a 27, the second time I got a 31, and in between i bought a book but then didn't even look at it. Take from that what you will. I'm not quite sure what to make of it myself, whether the questions on the second one were that much more appealing to me, or I had the benefit of taking it a second time, regardless, that 31 really bolstered my college app!
The packet comes with a sample test, right (I took it over 10 years ago, so I'm not sure)? Take that a few day ahead of time just so you know how the test is structured. This ain't boards or the bar. You'll be ok.
Plus with the ACT you can take it as many times as you want, and not have to send the colleges any scores (at least thats how it was when I was when I took 4 years ago). So take it once, and if you don't like your score take it again. You will be much more prepared.