All the major test prep books for the SAT, ACT, and GRE -- published by companies like Kaplan, Princeton Review, Barron's, and Manhattan Test Prep -- are poorly written, conceptually deficient, and, worst of all, riddled with serious errors. Students can't be expected to learn from books that aren't even right! And I don't mean the books are riddled simply with typos, which unfortunately is also true, because they are so poorly edited; I mean they really are riddled with serious conceptual errors.
Here's a simple example from the Introduction (page 23) to Manhattan's Strategy Guides for the Revised GRE. This passage appears in all eight of Manhattan's strategy guides, so it somehow went unnoticed after at least eight rounds of editing by allegedly "expert" readers and test-takers. See if you can spot the error!
"If ab=|a|x|b| which of the following must be true?
II. a>0 and b>0
A. II only
B. III only
C. I and III only
D. II and III only
E. I, II, and III
Solution: If ab=|a|x|b|, then we know ab is positive, since the right hand side of the equation must be positive. If ab is positive, however, that doesn't necessarily mean that a and b are each positive; it simply means that they have the same sign.
I. It is not true that a must equal b. For instance, a could be 2 and b could be 3.
II. It is not true that a and b must each be positive. For instance, a could be -3 and b could be -4.
III. True. Since |a|x|b| must be positive, ab must be positive as well.
The answer is B (III only)."
In fact, none of these statements must be true, so the question, as written, doesn't even have an answer! (You can easily see that III need not be true if you choose a or b equal to zero.) The serious conceptual error is to assume that "the right hand side of the equation must be positive." See if you can't add something to the question to make the correct answer be B -- or, alternatively, try tweaking the options labeled by roman numerals so that one of them becomes correct.
This is just one particularly glaring example of an unfortunate trend. Don't waste your money on prep books that are poorly edited and often seriously confused. Hire an expert tutor, instead!