I keep seeing people recommend the usual suspects like 'The Shining' or 'House of Leaves,' which are fine, but they don't unsettle me the way a more clinical approach can. A book that got under my skin recently was 'I'm Thinking of Ending Things' by Iain Reid. It's short, almost deceptively simple, but the entire narrative feels like a trapdoor giving way under your feet. The paranoia isn't about a monster in the closet; it's about the unreliability of every thought in your own head. You finish it and just sit there, questioning the dialogue you just read, which is a far more potent kind of scare.
For a different flavor, 'We Need to Talk About Kevin' by Lionel Shriver is a slow, dreadful dissection. The horror is entirely in the psychological dynamic between a mother and her son, in the ambiguity of nature versus nurture, and in the chillingly mundane lead-up to an atrocity. It's not about jump scares; it's about the grinding anxiety of watching a catastrophe form in slow motion, made worse because the narrator's own perspective is so bitterly flawed.