A Scanner Darkly by Philip K. Dick

Bob Arctor is a dealer of the lethally addictive drug Substance D. Fred is the police agent assigned to tail and eventually bust him. To do so, Fred takes on the identity of a drug dealer named Bob Arctor. And since Substance D--which Arctor takes in massive doses--gradually splits the user's brain into two distinct, combative entities, Fred doesn't realize he is narcing on himself.

Caustically funny, eerily accurate in its depiction of junkies, scam artists, and the walking brain-dead, Philip K. Dick's industrial-grade stress test of identity is as unnerving as it is enthralling.