For both the SAT and ACT I recommend review books from the companies that make up the tests. For the SAT, you’ll find that information at www.collegeboard.com and for the ACT, www.act.org. In my classes I provide the Official SAT Study Guide, the college board review book that includes 10 practice tests. Some of these 10 tests are actual previous SATs and all of them are the most accurate and far better to practice from than those in any other review book. At this point I will take the college board to task for their ridiculous logic. On the one hand, they tout how much students can improve from using their review books and on the other hand they want to say that prep classes don’t help students that much. Well, prep classes often use the college board review books (as my class does), motivate students to practice from these books, and in addition provide the tips and techniques to help students in approaching all the questions. So how can the books themselves help, yet the books with all the other benefits of a teacher not help? Where is the logic in that? I just have to shake my head at such nonsense.
Other review books can be valuable in their explanations and may present materials in a fun way that could be helpful, but doing practice questions is the most important preparation and the Official Study Guide is best suited for that.
For a longer term preparation, I recommend vocabulary training. Most books that say SAT vocabulary on them include virtually the same words but present them in different ways.