Okay I personally would NOT go with the workbooks if I were you, unless you are seriously struggling with a particular area of the SAT. I own the "Cracking the SAT" princeton review book and think that it works just fine. You may feel that the book is a bit informal at the beginning, but stick with it and read it dozens of times and you'll be fine. I think that the Princeton Review does an excellent job with test prep- they helped me score within the top percentile on my PSAT and I'm planning to take private tutoring with them in the summer.
In all honesty, though, I say that because really that is the first thing I picked up and it worked for me. But I've used Kaplan too and found it just as useful. People get so worried over the whole "princeton review versus kaplan" thing that they don't stop to think that each one helps in its own way. I think that Kaplan just stresses you out about the test more. I also really like the princeton's review's service-my tutor not only gives me great college advice but also seems to know my weaknesses so well it's creepy. ; )
Stick with whatever appeals to you (if you're looking for some fast, score-raising tips go with P.Review-- if you want a really vocab-based study plan, i agree that kaplan does a great job with that).
And about the subject tests-- i wouldn't really worry too much about those books. it's a waste of money; do well in your core classes and just get something to get your high/perfect score. they really are not that difficult.
Oh yeah and one more thing- I agree that all the "Official Guide" books are extremely helpful, especially if you are cramming, because they are really to-the-point and have lots of practice drills.
I have to admit that the "Cracking the SAT" book from the P.Review has only 3 practice tests in it, but if you are buying it go ahead and get the "11 practice tests" book with it. But don't do what I did and use them all up quickly-- the 11 practice tests tend to be somewhat similar so give yourself a break in between them. That was my testing downfall.
Well, that's all I have to say
There's no PERFECT book out there- it really all depends on your ability and dedication. the book can only do so much.