The most important book to practice from is The Official ACT Prep Guide, which comes from the test makers themselves.
For additional theory and practice we would recommend the following:
- Kaplan ACT Premier (2016-2017) which comes with 8 fairly challenging tests
- Princeton Review: Cracking the ACT test with 6 practice tests. This perhaps the most realistic simulation of the real ACT test and so is highly recommended for practice.
- Barron’s ACT. This is a little over the top in some areas as far as difficulty levels are concerned. For the students that are aiming high this is a must.
- McGraw-Hill 10 ACT Practice tests. And just for pure extra practice, this book serves up relatively realistic practice tests, 10 of them to be exact.