I don't mean books that have just made you a little uncomfortable either, but the books – novels, graphic novels, non-fiction, short stories or poetry – that have rattled your cage so much that you daren't read it again and you even needed a breather afterwards? (Of course I don't mean this literally, but I'm sure you get what I mean). Feel free to post one book or multiple books. Whether it's because the violence was a little TOO detailed or the book hit some psychological nerve a little too close to home, let's share the books that have rattled the cage so to speak.
For me, I have a few books I could consider for this:
Ryu Murakami's In The Miso Soup – A Japanese man escorts an American tourist around the hot-spots but this American is gradually revealed to be a serial killer. Ryu Murakami never shies away from violence in his books and this one is no exception. I felt like I needed a shower after having read it.
Junji Ito's Uzumaki – A whole town becomes obsessed with spirals to the extent that it consumes everyone and everything. I'm not exaggerating, the book gets kind of ridiculous with it. It consists of chapters that have self-contained short stories and some of them are visually shocking.
Cormac McCarthy's Outer Dark – I feel Blood Meridian would be the more obvious choice but the ending to Outer Dark has stuck with me for longer than the violence in Blood Meridian (although Blood Meridian is incredibly disturbing too).
Jonathan Swift's A Modest Proposal – An incredibly short satirical essay written by the author of Gulliver's Travels. He proposes the concept of solving poverty and hunger in Ireland by encouraging the harvesting of vagrant children for cannibalism. It's a very short read and it starts out as darkly comedic but it becomes more disturbing with the details Swift goes into (not gratuitous violence but he rationalises it in a way that's quite realistic if such a proposal was taken seriously, to the point where it is no longer darkly comedic and you have to wonder if he was actually joking or not – of course, he probably was, but it doesn't lessen how disturbing this "proposal" of 4-5 pages is).
TL;DR: what are some of the most disturbing books you have read? What did you find disturbing about them?