Best SAT Math Prep Book for Low Scorer?

I'm scoring below a 400 on the SAT Math section, and I'm really worried. I would like to get my score up into the 600s if possible. What's the best prep book for me to use to improve my score as quickly as possible?

I would recommend 28 Lessons to Improve Your Score in One Month - Beginner Course. This book is specifically designed for students scoring 500 or lower on the SAT math section. You can get it here. Once you start earning higher scores, you can transition to the next book in the series for intermediate students. You should also use the questions on the Khan Academy website for supplemental practice.