I've plugged my friend Chiron's blog before; he writes about violence and martial arts training. And generally about learning things--I don't have a guru but if I were trying to find one, he'd be in the running. I am lucky to be blessed with his friendship.
He has a lot to teach. You can read some of it on his blog, where he posts about the problems of training law enforcement officers to deal with real violence, and the traps martial arts training can set, and the way criminals think and act. You can also read about him at his website. And if you're very smart, you will buy his book, Meditations on Violence: A Comparison of Martial Arts Training & Real World Violence. (That's an Amazon link; feel free to buy it another way, I don't get a commission or anything.) It will be published in June 2008, and I am really looking forward to it.
If you're a fiction writer, you will learn useful stuff about real-world fights and violent attacks, and the emotional responses to them. If you're in the field of law enforcement, you'll read his insights about dealing with jail inmates and other violent people. Martial artists might want to argue with his ideas on how your training can handicap you in a real-world fight (and they do, on his blog--but my money's on Chiron). For anybody, this book will give you a perspective you probably don't have about casual violence, the monkey dance of group violence, the blitz attack of the mugger or rapist, and what is possible in response and after the event for your survival and recovery. I recommend it highly.