If you want to hear Philip K. Dick himself discuss the writing of A Scanner Darkly and describe some of the borderline-paranoid ideas that drove the plot, there’s a three minute video clip on YouTube.
There’s evidence that Dick had reason to be paranoid. It is likely that he was investigated by the authorities during the period of anti-communist McCarthyism because of his anti-government views.
The burglarly he talks about is mystery, and it is not clear whether he was burgled by secret services, drug-using associates or whether he did it himself during a period of psychosis.
The fact that all three are possible candidates says much about Dick’s life.
UPDATE: NPR Radio’s Talk of the Nation has a fantastic discussion on PKD’s life, work and influences.