I think it depends on the person. My score went up from an 1180 to a 1400 with a class. Though I had great grades in school, as well as university, I'm just not hardwired for testing situations (I'm more of a research person). The class helped me understand how to work the test (which, let's be honest, is all an SAT is - it has no basis in measuring actual intelligence).
Basically, like most other things, it depends on the kid in question. In my opinion, if you want to know how what types of words are related to others, how to figure out some equations on the test, etc... then I think a book is fine. If you want to know in what situation you should skip a question, when you should guess, how you should work through the test in the most efficient manner... testing techniques basically, I found the class helpful.