<p>The best ACT prep book that I've found is "The Real ACT Prep Guide" by the makers of the actual test. It comes with three practice tests, and explains pretty well how to look at the science portion(as well as the other sections obviously). You can order it from the ACT website if you're interested.</p>
<p><a href="http://www.act.org/store/index.html%5B/url%5D">http://www.act.org/store/index.html</a></p>