The novel shifts into Philip K. Dick territory, and then shifts further into the sort of metaphysical science fantasy that Colin Wilson hit people on the head with back in the 1970's in bait-and-switch novels like The Space Vampires and The Mind Parasites. The philosophies espoused here are much more palatable than those in Wilson's novels. Moreover, Shirley keeps his characters fallible and the ground-level stakes in view. The scenes of horror towards the end of the novel are more Zombie Apocalypse than Demonic Invasion, but they're repeatedly framed in terms of human loss and sorrow.
I liked Demons a lot. Your response may vary depending on how much mystical lunacy you're willing to withstand. Recommended.