The best study guides, far as I can tell, are the ARRL books
http://www.arrl.org You will find a link to the "Store" in the upper right portion of your screen.
When I first went to Quartzsite I picked up a Gordon West study guide. This is a simple memory book, He lists all the possible questions, their answers and some hints toward how to remember the answers... But no clue as to how to DETERMINE the answer or to use the info. I very quickly slammed up against the old stone wall.
I then decided to take a class. The class used the ARRL guide... The ARRL guide includes the theory behind the answer so it kind of took a wreaking ball to that stone wall... And the instructor I had. Ground up the stone, mixed in some other stuff and poured out nice smooth pavement to pave my way to EXTRA class. I mean, that test was EASY and I'd only compleated 2/3 of the course (Plus some additional hitting of the books, I mean for a couple of weaks before the test date I lived, ate, breathed, slept, dreamed those tests... Passed too I did, 1st try
Now that was extra. There are only 3 classes these days, Tech, General and Extra
You will need to know some theory, some regulatoin and some operating practices to pass any of the tests.. Technican is more regulations and op-practice, general and extra add other elements.
For example.. Astrophysics (part of the extra is, in fact, rocket science, Very simple rocket science, but rocket science non the less)
You can see a sample test at
http://www.qrz.com I took perhaps a hundred of the extra class sample tests on my road to a license... passed every one too.