Reddit reviews American Legal History: Cases and Materials, 4th Edition

Are you an American? If so, to truly understand our Constitution (the basis of all our law) you need to understand Locke. I can't recommend a specific book, but I'm sure you can figure it out.

For more understanding on the American system of law, I also recommend the Federalist and the Anti-Federalist Papers. The Anti-Fed papers are arguably more important, since the Anti-Feds gave us the Bill of Rights. But the Fed Papers get all the sexy press.

For a better understanding of US Criminal law, you might want to read Bentham and Kant.

Specific primers you might try are: America Legal History: Cases and Materials, and Cases and Materials on Criminal Law.

Note that most legal treatises are ungodly expensive and mind-numbingly difficult to read. You might want to get some tuition on reading court opinions before embarking on your journey--it's not as easy at it (should be) seems, and if you only read other people's analysis...well, that's like getting your news from FOX or MSNBC: you always have to wonder about the spin.