When it comes to prepping for standardized tests, I'm a big fan of the Princeton Review books.
If you take sample exams for practice, use real former exam questions available from the
testing agency.
(Some of the prep books contain sample exams with question types and content that do not occur on the real exams. Unless you're using test prep as a way of generally reviewing a topic, why bother studying things that won't be on the test? I'm quite cynical about US standardized tests: I think they're stupid and pointless and I have no issue prepping so specifically for them.)