Ugh, I have so many prep books, it better be worth it! I'm taking Kaplan's online course, so I have those books. I also have the EK series, which I felt was ok for bio and chem, but didn't work for me for the other subjects. I also have the NOVA Physics book, which helped me a lot when I was really struggling with physics (7 years since I took Phys I and 5 years since phys II). Finally, I have the "Organic chemistry as a second language" books, which were also a great help since it's been 6 years since I took organic chem in college.