Law is a graduate degree, at least in the states. You can't test out of anything. Some colleges may have some law courses, but I doubt they allow testing out of them in general. Check on the specifics.
In general you'd be better served learning math, history, English and science really well in high school. Worry about law when you start on your JD.
Also, law schools don't really use textbooks. They have casebooks which are just court cases that are edited down a bit and all re-printed in one big, expensive book with a few notes from the editors. I'm not sure they'd be very useful for self-study. There are commercial outlines for courses available that, when combined with a relevant case book could allow you to get a general idea on your own. Maybe the best to start with would be Glannon's Civil Procedure Examples and Explanations. Check half.com for old editions, otherwise prices will be pretty extreme.