<p>What is the best book to use for preparation?</p>
<p>Definitely the official book, the ACT I took was very similar to it actually, I even remember a problem so similar on the practice I took the night before and it ended up being in the same position as the real test except with different wording (was still hard though :/).</p>
<p>Previously released tests are the way to go.</p>
<p>If you're going to take both the SAT and the ACT, I found it is better to study mainly for the SAT and a little for the ACT (to familiarize yourself with the format). The SAT, imo, is harder and challenges your reasoning skills more than the ACT. This preparation will spill over to the ACT and will in a sense overprepare you. All you need to worry about is the little extra math content and timing, which shouldn't be difficult since the problems take less time than SAT problems. Don't get too cocky though, I finished the math on a practice test with 10 min to spare, so I took my time on the actual test and couldn't finish the last 5 problems.</p>