I highly recommend "The Real ACT Prep Guide" which is the red book. It's the only book published by the makers of the ACT. I got a 32 the first time I took it in April, then I used the book and boosted it to a 33 for June's test. I did practice tests throughout the summer in the book and I received a 35 on October's test. I noticed that, especially with the math, the practice tests in the red book are extremely similar to the real ACT. Same basic problems, just different numbers. The best way i found to study was just run problems again and again, but make sure you time yourself for the practice tests. The hardest part of the ACT is the timing! Good luck! Feel free to PM me if you have any questions.