Hi Henry! I think Kaplan is a very good series to purchase. There are many study books out there but I personally liked Kaplan because it is organized in reviewing the concepts. There is also NextStep, ExamKrackers, and Princeton Review. I used Khan academy in my studying and I feel that the Kaplan readings aligned well in the videos for Khan academy. You can always research a bit and see how you feel before making a purchase especially since each test prep book is costly.
Best of luck!