(from the back cover) Cops and criminals have always been interdependent, but no novel has explored that perverse symbiosis more powerfully than A Scanner Darkly. Bob Arctor is a dealer of the leathally addictive drug called Substance D. Fred is the police agent assigned to tail and eventually bust him. To do so, he has taken on the identity of a drug dealer named Bob Arctor. And since Substance D - which Arctor takes in mammouth doses - gradually splits the user's brain into two distinct, combative entities, Fred doesn't realize that he is narcing on himself.

Caustically funny...may be the most unnerving drug novel ever written.