Here are three books I am recommending for the 2012 ACT and their lowest prices as of January 19, 2012:
1. Kaplan ACT 2012 Premier is $17 plus $4 shipping on amazon.com. If you buy over $25 in books, media mail shipping is free.
2. Princeton Review's Cracking the ACT, 2012 Edition is $11 new on amazon plus free media mail shipping, if you buy over $25 in books.
3. Real ACT 2011, 3rd Edition with CD-ROM and online resources is $17 plus $4 shipping on half.com.
My recommendations are based on student ratings, popularity, and pricing information obtained from multiple ACT-related websites, amazon.com, ebay, and half.com.