Most non- College Board test prep material is downright bad, as in, beyond "not helpful," veering into the "detrimental" zone.
Think of it like this: If you're training for a tennis match, you don't train on a squash court!
Not all unofficial material is "bad." In fact, some of it is excellent. The problem is in deciphering the good from the bad, which is not a job for the layperson who is studying for the SAT crushed by the millions of options.
"Best SAT Test Prep" = 2,980,000 results on Google.
So how do you know? Don't assume "hard" necessarily equates to "good."
5 Signs of "good" SAT test prep:
1) Refers back to the The Official SAT Study Guide (aka the Blue Book). Helps student interpret the Blue Book.
2) Written by an SAT expert. (Don’t assume PhD signals “SAT mastery.” Maybe,but maybe not.)
3) Recommends official material for diagnostic SAT, and full official practice tests as part of test preparation.
4) Has a goal-setting strategy: For sections that are in “order of difficulty” you should strive for a mastery of questions inside a goal zone before attempting harder questions. Put simply: you should leave hard questions blank if you haven’t mastered the easy ones.
5) Addresses issue of fundamental skills and test strategy. Exception: test prep aimed solely at high achievers.