Philip K. Dick The Three Stigmata of Palmer Eldritch (1964)
Palmer Eldritch is reckoned by some to be Dick's greatest, although having said that I can't even recall quite where I read such claims, and personally I'm not convinced. Not that it isn't an exceptional novel, but for my money he wrote at least three that were probably better for having more of a sense of humour; although to be fair, Phil wrote Palmer Eldritch almost immediately following a week spent under the terrifying gaze of a vast metal face staring down at him from the sky, so it's probably forgiveable that the funnies weren't coming quite so thick and fast as usual.
In an afterword to his short story The Days of Perky Pat which itself feeds into The Three Stigmata of Palmer Eldritch, Dick described the experience thus:
There I went, one day, walking down the country road to my shack, looking forward to eight hours of writing, in total isolation from all other humans, and I looked up into the sky and saw a face. I didn't really see it, but the face was there, and it was not a human face; it was a vast visage of perfect evil. I realise now (and I think I dimly realised at the time) what caused me to see it: the months of isolation, of deprivation of human contact, in fact sensory deprivation as such... but anyhow the visage could not be denied. It was immense; it filled a quarter of the sky. It had empty slots for eyes, it was metal and cruel and, worst of all, it was God.
I drove over to my church [. . .] and talked to my priest. He came to the conclusion that I had had a glimpse of Satan and gave me unction - not supreme unction; just healing unction. It didn't do any good; the metal face in the sky remained. I had to walk along every day as it gazed down at me.
It should probably be noted that Dick was reputedly yet to try LSD, despite what assumptions might be drawn from the hallucinogenic drugs Can-D and Chew-Z which feature so heavily in the novel; but regardless of cause, the experience was clearly absolutely real for him, and this novel was an attempt to deal with it, to reconcile the encounter with the rest of his developing cosmology. So the face in the sky became Palmer Eldritch, the degenerate God infesting the hallucinatory realities of those who consume the drug he has brought back from Proxima Centauri.
If not his best novel, The Three Stigmata of Palmer Eldritch distinguishes itself as Philip K. Dick with gloves off and cards on the table more than anything he had written previously. Here his view of the universe as roughly informed by gnosticism is clearly expressed without having been smuggled out inside a different story. It carries all of those signature elements which characterise a Dick novel, and all fully realised - the dark or otherwise mad Deity returning from afar, the struggle to escape from the dismal and false reality brought to the world by the errant God, with only the dry humour not quite up to its usual strength. After Palmer Eldritch, Dick recovered the use of his chortle glands and went on to write better novels, but this one is still astonishing, its only failing being that there was better to come.