If you believe you are by now immune to gory novels, here’s one with enough malevolence to give even the most hardened readers nightmares. “The Seven Days of Peter Crumb,” a chronicle of the final week in a psychopath’s life by the British actor and writer Jonny Glynn, is gruesome, obscene and utterly disturbing. It is also absorbing and well written. Reading it, I fought the urge to throw up. Needless to say, I was transfixed.