It's the story of a man who is troubled by voices in his head, but who hasn't been able to get much help from the medical establishment because his symptoms don't fall into any of the accepted models of demonic possession. Y'see, the world of this book isn't *exactly* our world, though it is very similar. The issue of demonic possession is clearly real, though lots of scientists and doctors remain confident that they can find rational explanations for the phenomena. It's a trope most frequently seen in fantasy or horror, but it's treated here in a very straightforward, almost SF-nal way. I cannot, at this point at any rate, say definitively what genre this book is.
The author is clearly aware of the genre ambiguity, and not interested in making it any simpler. At one point it actually gets foregrounded in the text. The protagonist is peripherally involved in a conversation about the difference between SF and Fantasy. Among the claims put forth is that in SF, the characters believe that the world is rational and understandable, whereas in Fantasy, magic is simply taken for granted and regarded as unexplainable. Characters in this book fall into both those categories (and many more fine distinctions), and it's far from clear yet who is 'correct' (if, indeed, any of them are).
By the way, one of the characters involved in that genre conversation was, depending on your own point of view, either a man pretending to be possessed by a demon, or a demon pretending to be an eccentric old man. And not just any old eccentric: he's either a Philip K. Dick who lived to be older and madder than he did in our world, or a demonic entity named VALIS who is animating Dick's body after it actually did die in 1982. And that much should have you convinced to either run out and buy this book, or to avoid it like the plague, according to taste :-)