A Scanner Darkly (Millennium SF Masterworks S)

Substance D -- otherwise known as Death -- is the most dangerous drug ever to find its way on to the black market. It destroys the links between the brain's two hemispheres, leading first to disorentation and then to complete and irreversible brain damage. Bob Arctor, undercover narcotics agent, is trying to find a lead to the source of supply, but to pass as an addict he must become a user, and soon, without knowing what is happening to him, he is as dependent as any of the addicts he is monitoring.