One of the most useful resources I have discovered in my pursuit of accelerating my degree through CLEP tests is InstaCert. This website provides study materials that are created specifically for these exams as opposed to broad subject textbooks. The main resource they provide is a collection of hundreds of flash cards per each exam, which makes the studying process much more active.
If I remember correctly, I believe that it is around $20 a month to subscribe to InstaCert. I am sure that I could provide you with some code with a discount and get some sort of kick back. However, that is not my intention with this website. My intention with this website is simply to spread information and help as many students out as possible.
In my opinion, InstaCert is the most efficient way to study since you can go through the flash cards rather quickly. You can also pull the flash cards up on your phone, so if you just study 30 minutes throughout the day, that means in no time you will be ready to sit and pass that test. Let me know if you have any experience with InstaCert and what you think about it!