Best ACT prep book?

I’m most likely going to take the ACT in April and I’m aiming for a 30+. What is the best book to study with? Should I get a book that’s harder than the actual ACT so that my practice test scores are lower than my real score? I’m particularly bad at science and math so is there any books with good math/science hep?

For my first time taking the ACT I used Princeton Review’s Cracking the ACT. My practice test scores from Princeton were almost the exact same as my actual scores, only a 1-2 point difference per section.

I used this book and it helped a lot http://www.amazon.com/lb-Book-ACT-Practice-Problems/product-reviews/194123450X