I did that. I used most of them and got to my target score within a few months. I think it was more that I was really nervous--and lacked the skill set. Each book had its own separate perspective on HOW to do different things and that really came in useful. I used them for the SAT Writing/Math. But I couldn't get through the reading passages in the extra reading ones I bought. Too boring.
Realize, that those of us buying the books may not be taking expensive SAT courses and are using these books as our strategies in lieu of some $3,000 course that takes up more free time than reading the books for 30 min a day or so.
Between my PSAT/SAT, I knew that for the colleges I was interested in, I needed to dramatically boost my score by over 300 points. And these books gave me the ability to do so. The way I figure it: $20 here on a book I skim over is much cheaper than $3,000 in a course I sleep in.
I actually gave my unused books to a friend to have so they could save money.