jim's advice above is good: go with the official guides first and foremost, because they're the official guides. they are "the native speaker", so to speak, and so their idiosyncrasies, preferences, and rules will be 100% reflective of those on the real exam. when you buy materials from other companies - especially mass-production outfits such as arco and rea, which produce books for everything from military flight tests to nurses' training - you run the risk of internalizing rules, preferences, or material that are irrelevant to (or, in the worst cases, in contravention of) the official ones.

if you find yourself wanting to buy a huge quantity of books, then you may well have the wrong perspective on how to study for the test: specifically, you may be wrongly emphasizing quantity over quality.
good studying should be focused on quality, not quantity. the goal is not to get through as many problems as possible; the goal is to treat problems thoroughly. to that end, you should study not only the particular solutions to the problems, but also the connections between those problems and any other, similar problems you've already studied.

here are some issues you should be considering, at least in passing, on every problem with which you have difficulty:

A. Foundational Knowledge:
1. Did I COMPREHEND the symbols, text, questions, statements, and answer choices?
2. Did I understand the CONTENT being tested?

B. Recognition:
3. Was I able to CATEGORIZE this question by topic and subtopic?
4. Did I make a CONNECTION to previous experience?

C. Execution of Primary Strategy:
5. Did I choose the best APPROACH?
6. Did I have the SKILLS to follow through?

D. Secondary Strategies:
7. Am I comfortable with OTHER STRATEGIES that would have worked, at least partially?

E. Traps & Tricks:
8. Do I understand every TRAP & TRICK that the writer built into the question, including wrong answers?

F. Assessment & Retention:
9. Have I MASTERED this problem?
10. Will I REMEMBER?


if you consider all these issues, you'll find that the official guides + perhaps one other company's materials is more than enough for your preparation.