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!