I used Mosby's Review for the Pharmacy Technician Certification Examination, 2e (some of the questions, such as drug classifications have changed so just be aware of that). The practice tests were still really useful.
I also used Certification Exam Review for the Pharmacy Technician (3rd Edition) (Mike Johnston). This book had updated drug classifications.
I studied for a couple months over the summer, with an average of an hour/night and passed with no problem!