First, read everything CT, Waterbury, Cressey, Robertson, etc have written on this site. Some of it conflicts, but it will give you different points of view and a pretty good all around base of knowledge. Best of all, it’s free. There are hundreds of thousands of books out there, you might want to pick up a college level anatomy textbook if you’re really serious.
Beyond that, maybe look into the home study courses offered by most certification institutes.
NSCA/CSCS study guides: http://www.nsca-cc.org/online_store/products.html?ci=1
ACE study guides: http://www.acefitness.org/getcertified/prepare_ptmaterials.aspx
I would recommend the CSCS study guides over the ACE.
You could also pick up any of the books from the Biotest store, or anything by Mark Rippietoe. I’m sure some other people can give you some more specific reading recommendations.