There are many, but my absolute favorite is Joy Williams'
The Quick and the Dead (no, nothing to do with the Sam Raimi film). Nominally it's about three teenage girls - acid-tongued eco-warrior Alice, grief-stricken orphan Corvus and shallow rich girl Annabel - drifting through summer in a desert town, but it's not really about that at all: it's actually about The Big Themes, life and death and the sometimes malleable divisions between the two, ghosts and madness, signs and symbols. It's an absolute goddamn masterpiece, is what I'm saying.
My second favorite: Mervyn Peake's Gormenghast Trilogy (fine, that's technically three books, but I read 'em in one volume, so nyaaah). There's really no way to even begin summing this one up: it's a bizarre, grotesque Gothic fantasia, the anti-Tolkien, High Fantasy with no magic whatsoever, only some of the strangest and most memorable characters you'll ever encounter and a prose style that goes beyond ornate into pure surrealism.
And, last but not least, my favorite author is Joyce Carol Oates, who has written such an insane number of books and stories that it's probably easier to just count her entire oeuvre as one thing, but her most famous work is probably the much-anthologized short story "Where Are You Going, Where Have You Been?", which is brilliant and chilling and can be read here:
http://www.usfca.edu/jco/whereareyougoing/
