A Scanner Darkly

Here is PKD in his classic mode of paranoia and questioning of reality (whether a reflection of his true persona or used as a plot device, it doesn't really matter, since the book is tightly written and entertaining in its confusion, as is usual for PKD).

Fred is an undercover police officer whose assignment is to investigate Bob Arctor, who is severly addicted to Substance D, a brain-damaging narcotic. The main plot point is that Fred and Arctor are the same individual, but he is unaware of this secondary to the effects of the drug and how it has essentially split his personality. A very unique narrative POV and look into the mind of a drug addict. As with most of his work, the ending is an unhappy twist.

Not PKD's best work, but still very much worth reading.