WHAT TO LOOK FOR
Consider how a particular book teaches one who is self-studying. For many people, learning something from a book without a teacher can be a difficult experience. If this is true of you, then you may want to find a book that will guide you through the material well. Are you seeking extensive review with a serious approach to scoring high? Or, are you having difficulties comprehending the basics of the act and need a simple book? Base your decision on these personal factors, rather than which book others consider "best". What may have worked well for someone else, may not necessarily work well for you.
WHAT NOT TO DO
Buy more than one commercial test prep book. Working through multiple books won’t help more. Indeed, it will actually hurt, as you’ll spend too much time learning methods and not enough on practice tests. Pick one book and stick with it.
SHOULD YOU TAKE AN ON-LINE COURSE INSTEAD OF USING A BOOK?
If you think a preparation book is not enough, but are unable to take a live course, consider taking an on-line course, which often provides essentially the same content as a live course would, yet via on-line. Many on-line course options provide you with an e-mail contact if you get really stuck - a big advantage over a book. If you're okay without a teacher but want much more support from what a book provides, consider on-line options.