i think the best book you could get him would definitely be IN THE GRAVEST EXTREME by Massad Ayoob. its really well written and informative. i couldnt imagine any better book on the subject to give to your buddy. in fact, i think anyone who CC's can benefit from this book.
another one i read not too long ago that is would also be perfect for what you want is ARMED RESPONSE by David Kenik. it may even be a little bit easier of a read.
they are very similar books, and i personally think that anyone who even so much as owns a gun should read these books. and i say that because these books would be an invaluable tool in the event , god forbid, you ever have to use a gun for self defense.