It is HIGHLY suggested that you purchase one of the books below to prepare for your AP Test in May. It can be the difference between getting college credit or not, so it is worth the $20. It can also help you study for the class (although not required). You can get an electronic version cheaper but I believe that writing in the book is very helpful.
You can purchase the book online from Amazon, Barnes & Noble, and even Walmart carries it. I like the Barron's version below BUT MAKE SURE THAT YOU GET THE CORRECT EDITION! (The edition matters because the test was changed a couple of years ago).
Either of these books are fine:
Some students also like the Flash Cards as a way to study (you will probably see them when you search for the book). These can be helpful, so are suggested, but the book is just fine WITHOUT the cards. Up to you!