I really enjoyed this quick and easy-to-understand guide to St. Paul and his writings. Well, perhaps "easy-to-understand" isn't fair, because the topics covered here on the writings of St. Paul are not easy ones. Yet this is a reasonably accessible place to start and might even be usable, with some guidance, for a teen Bible study group.
I've always been a fan of good writing in small packages (in this case approx. 4 x 6 inches) because not only can I get to the heart of the matter fairly quickly, but also conveniently as the book can be stashed in my purse.