To study specifically for the ACT (or SAT, SATII, etc.), you should pick up a study guide specifically for that test. There are many to choose from. Those from Princeton Review are popular.
College texts are another matter. If you want to get a head start on college physics, you might pick up a copy of Halliday and Resnick. If you want to go cheap, pick up a used copy from the 70's or 80's (it was 2 volumes), the physics is unchanged. Make sure to study calculus (with vectors, Gauss's theorem, etc.) at the same time if you don't already have a solid grounding.