If you're going to buy just one book, buy the later, if you want to get one more buy the first one.
Also check out the NAR website, as there are practice tests on there, just work through those a couple of times - making sure that you really understand what's being asked, and that it's not just straight memorization.
It's one thing to know that the CG has to be in front of the CP, it's another thing to know WHY....
I would not rely on Stine's Handbook of Model Rocketry to study for a Level 2 test. It has some great info in it, but I don't think it's the right recommendation for the test.
NFPA 1127 (viewable for free on the NFPA website) is a good reference, as is the Safety Code of whichever organization you plan to certify through. NAR & TRA tests are different, so what will help you depends on whose test you plan on taking.
With Tripoli, you receive a Study Guide with your membership binder which contains everything you need to know to pass the test. I'm confident NAR has similar information available; I just don't know where to point you to find it (I took the TRA test).
Marc Canepa's book is something I've not seen myself, but I've heard nothing but good things about, so I do think that's a great "getting started with HPR" book that will be a worthwhile investment.
What's your background and experience level, and what organization do you plan to certify through? We can better direct you with that information.
If you are going to do the Tripoli level 2, go to the Tripoli site http://www.tripoli.org On the menus, click Documents, then Certification, then How to Certify, then Practice Test. This will take you to another site that will let you download a practice test. This test has all the possible question that can be on the L2 exam. Since you download it to your computer, you can take practice tests anytime, if you are online or not.
I second the recommendation to study TRA's safety code, found right on their website. Plus, when you join TRA, your packet will include the study questions. Do what I did and make flash cards for each question and just review them when you're sitting on the throne. You'll get it down pat in no time!
Mark Canepa's Modern High Power Rocketry 2 is an awesome read. Even if you've been flying for a while, there's still quite a bit of information to be had. It's a great way to get the basics down and learn a few advanced techniques as well. It belongs on every rocketeer's bookshelf.