Blood Meridian - Cormac McCarthy. I started it a few years ago and thought it was just grimy, but gave it another go while I was in hospital. There is little in the way of a plot, just an exercise in seeing how much gratuitous sadism, obscene cruelty, violence and abject misery can be packed into every page. It seems to have gained notoriety because of this and is even described as a masterpiece in some quarters, although it is very badly written.
If you're determined to read the most sickening book ever written it will probably fulfil your needs.