There's nothing I like more than an unreliable narrator (if you can believe that), and as far as concerns first-person protagonists who are somewhat detached from reality, James Lasdun's Lawrence Miller in "The Horned Man" knocks previous record-breaking barmpots like Patrick McGrath's Edward Haggard and Nabokov's Charles Kinbote into a cocked beret.
The blurb is reprinted above [ie on Amazon] so I won't go into the plot, but please let me assure you that it will not spoil your enjoyment to know now that Miller is as mad as a cake. It's pretty obvious from page one, where he discovers the placemarker has been mysteriously moved in one of the books in his office. But the delight of "The Horned Man," which brings it more in line with "Pale Fire" than "Dr. Haggard's Disease," is that although we have a fair idea of what's not real, we are never explicitly told what is actually happening. Even at the end (which, in another fine touch, neatly has us turning back to the beginning) Miller's meticulously consistent delusions will not let go. The normal way for an author to let the light in on dusty attic of a barmy narrator's mind is to interject remarks from real people, which Lasdun resorts to only once. Here you're pretty much on your own.
Of course it is a feature of such a turbulent and twisted narrative that you can't really tell too much about what happens without spoiling it, but I can safely reveal that Miller's odyssey takes in cross-dressing, unicorns, swingers, bloody murder, excrement on desks, and representations of female masturbation in "Mansfield Park". So don't say there isn't something there for you. The other joy is the prose, which is surprisingly lively and fast-moving for a poet but with all the careful beauty you would hope for:
"Night had fallen by the time I reached my block down between B and C. It had been a crack block when Carol and I had moved there a few years ago - vials all over the sidewalk like mutant hailstones; stocky, stud-collared dealers in the doorways with canine versions of themselves grimacing on leather-and-chain leashes; a false bodega with an unchanging display of soap powders gathering dust in the window and a steady stream of human wreckage staggering in and out through the door..."
"The Horned Man" will either become a modern classic or fade into obscurity within a few years. In which case I'm delighted to have (a) got in on the ground floor for once, or (b) caught it while it's still in print [Delete as applicable]. You should too.