A Scanner Darkly - Philip K. Dick's best book?

Substance D is a new drug sweeping the nation and slowly destroying the minds of its users. As the connection between the two halves of their brains degrades, they grow increasingly disoriented and confused before suffering irreversible brain damage. 

Fred is an undercover narcotics agent working to uncover where the new drug is coming from. But to find the source he has to pose as Bob Arctor, a user, and soon Arctor is as addicted as the junkies around him. Can he see through his psychosis long enough to tell which leads are real and which imagined? Or will he be consumed, like his friends, by substance D?

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