I dunno how it is in Texas, but here in Florida the book called, guide to federal pharmacy law by Reiss & Hall and the statutes from your BOP should be enough. Also, know everything about Controlled Substances b/c it shows up a lot on the exam and in practice.
Go to that site (I cut and pasted above) and you will be able to purchase a copy of the law book to study for your exam. My brothers moved to TX a while ago and they both studied that law book. It's about 500+ pages thick. They said that the exam was quite difficult compared to the ones they took in AZ and CO. I bought the 2005 Edition thinking that I would have time to study for it. Well, it's sitting on my shelf collecting dust. I'm still in CO. I'm not moving to TX anytime soon, so I don't have to take the exam yet. HA! Good luck!
p.s: Make sure you let them know that they don't need to send you a new edition every year. Otherwise, they will send you a copy every year and charge you for it.