A Scanner Darkly by Philip K. Dick

Substance D is not known as Death for nothing. It is the most toxic drug ever to find its way on to the streets of LA. It destroys the links between the brain’s two hemispheres, causing, first, disorientation and then complete and irreversible brain damage.

The undercover narcotics agent who calls himself Bob Arctor is desperate to discover the ultimate source of supply. But to find any kind of lead he has to pose as a user and, inevitably, without realising what is happening, Arctor is soon as addicted as the junkies he works among…