The book that profiles the author's descent into madness. He both narrates the story as himself and is also another character, "Horselover Fat", who whilst we are told he is the same person, interacts with the narrator as seperate person. Presumably indicative of PKD's own split personality disorder?
I don't know how much of this we are to take as real, or at least PKD's genuine belief as to what's real, but we can either take it as the whole world being insane with messages and signs of rationality that only a few are aware of, that are indicative of our impending release from our self imposed prison. Or this could be about what it's like when you suffer from mental illness, how you feel sane; it's just everyone else around you that seems crazy and how you will always be able see things around you in a way that re-inforces your paranoid fantasies.
I think PKD intended this ambiguity but I think he genuinely believed that it was in fact real, that he had had a genuine insight into the true nature of reality and, in laying down his tractus, was sharing his insights with the world. He just went into it in too much depth and was the intense focus of too much of the book to have been something that he didn't genuinely believe in.
The book started and ended well but too large a part in the middle just didn't make good story telling and was just him trying to get across his crack pot ideas. This was a real disappointment for me as I had really high hopes for this book. A Scanner Darkly is one of my all time favourite SF stories and I thought this one might be along similar lines. But I think PKD was just a bit too far gone over into madness when he came to writing this and unfortunately wasn't, for me at least, nearly as effective.