Yes, I took a few CLEP or similar exams, such as Humanities. They are quite challenging, but the passing rate is pretty low. You have a fighting chance to pass if you follow the right prep strategy.
You could find some prep books, but I would not rely entirely on those. Instead, go to the closest college or university bookstore and ask them to direct you to the most frequently used textbook for the specific CLEP subject. Get that book and study everything in it. An introductory textbook is usually going to be enough as long as you learn all the material.