I would get the "Real ACTs" book. It has 3 old ACTs so it is probably the most accurate of the test books. The ACT bulletin contains the 2003 ACT, so that would be good to download as well, besides it is free to download or you can get it from your college counselor.
McGraw-Hill has 5 tests, but a number of errors throughout. Princeton Review has bizarrely hard science problems, and some of the problems have to be errors as well. Outside of that, it has pretty accurate exams. I have not looked at the Kaplan or Barron's book.
If you need to review material, you can use your Math book as well as the answers after each of the exams in the Real ACT exams.
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