I don't necessarily mean professionally, or even at the amateur level. Could be just for fun or to stay in shape. If so, know any good books/sites/resources for a beginner? It's something I've always wanted to get into. I know you'd probably need to get a trainer to actually get "good" at it. But I'm not looking to box competitively or anything, just want to do it mainly for the cardio and to keep me on my toes. I'm talking me and a heavy bag in my basement type of stuff.