Philip K. Dick & Roger Zelazny Deus Irae (1975)
I'm unfamiliar with Roger Zelazny, although Deus Irae features a few passages written in a style quite unlike that of Dick, so I assume those would be his, despite which this collaboration feels very much like the work of the author of VALIS. In fact, it could almost be read as a run-up to that novel without too much shoehornery.
Deus Irae follows the pilgrimage of Tibor McMasters as he travels across the wasteland of a post-apocalypse Earth in search of Carlton Lufteufel, the author of the aforementioned apocalypse who is now considered an avatar of the God of Wrath. McMaster's is to paint the living image of the Deus Irae for his church, the Servants of Wrath; in other words, he seeks to establish the existence of God, or at least of this particular manifestation - essentially the mad demiurge who casts a shadow of illusion upon the world in Dick's roughly Gnostic understanding of reality, amounting to the same malign presence he experienced gazing down from the clouds for an entire week back in the sixties - a visionary interlude to which this novel refers in several places.
McMasters is followed by Pete Sands, a more orthodox Christian who disputes that Carlton Lufteufel can truly be regarded as a God and who views the pilgrimage with great uncertainty. Pete Sands is a fairly transparent author stand-in - Dick rather than Zelazny in this case - but it might be argued that the two of them, McMasters and Sands, share a relationship not unlike that of Horselover Fat and Dick himself in the later VALIS, two aspects of the same divided personality. Both seek God by different means, and it is Sands who relates the experience of the face of a wrathful creator seen gazing down from the sky. Furthermore, McMasters encounters a worm by the Biblical meaning of the term, a creature which anoints him with a slime that grants understanding of the language of the animals roaming the wasteland. It's difficult to miss the parallel with the Edenic serpent which grants knowledge, or how immediately following this incident, Pete Sands is similarly able to communicate with the beasts he encounters. Of course, the reason for this, as given in numerous summaries spread all across the internet, is that with this being a radioactive wasteland, these are mutant creatures which have developed sentience and the ability to speak; but if this explanation appears in the text, then I'm afraid I failed to spot it, and the encounters read more like visionary religious experiences; and they're actually more interesting as such.
This, for me, is true of the novel as a whole, and particularly in terms of whether or not the guy who blew up the world can be regarded as a God in any meaningful sense: read as post-apocalyptic science-fiction with the usual array of mutants, bunkers, and robots that have turned a bit wonky in the absence of programming, it's great, but you're really missing out on the good stuff, just as A Clockwork Orange isn't really about how snazzy it is to be a futuristic teenager. Read as one of Dick's convoluted religious allegories, I'd say it might even beat VALIS, aside from my not having read the later novel in a while.