I'm trying to decide whether to get Barron's ACT, Barron's 36 and Barron's Workbooks or Princeton Review and Spark Notes. What should I buy? Is Spark Notes a good ACT prep book? How do Barron's and Princeton Review compare to each other?
Of the books you mention, I would recommend Barron's ACT 36 as the best resource, especially if you're already a relatively high scorer. The Barron's book has more challenging questions (often too challenging compared to the real ACT) and is very through in its coverage of the material as compared to other review books. If you're new to the test or are less intense about studying, the Kaplan ACT prep book is also decent.
I actually think a better way to go, rather than getting any of those books, is to get the Real ACT Prep Guide and the ACT Black Book. The Real ACT Prep Guide comes directly from the ACT, so it has the most accurate practice questions. The only issue is that its answer explanations are pretty lame, which is why you should also get the ACT Black Book. The Black Book doesn't have practice problems, but it has a bunch of great strategies and answer explanations for questions in the Real ACT Prep Guide.