Which study books are worth it?!?! Please recommend!

<p>Hi, I'm planning on taking two subject tests in June, Math II and Chem, but I have no idea what books to purchase for studying. I would really like to do well, but I don't want to spend too much money. Please recommend some for me because I usually don't buy books for standardized testing. Should I just try to study off free online websites? </p>