Pet Sematary by Stephen King, but only because I was way, way too young to read it when I did so the first time. That book legitimately gave me nightmares for weeks and I was too scared to tell my parents why I was suddenly waking up in the night screaming. From what I've heard King himself considers Pet Sematary to be amongst his scariest work and even all these years later I'm inclined to agree. Funnily enough, I didn't find any of the movie adaptations to be even close to as scary as the book.

Terrify is perhaps too strong a word, but From the Fatherland, with Love by Ryu Murakami has kept me up at night while reading it. The setting for the story is an alternate history timeline in which Japan is occupied by an invasion force from North Korea. War in general terrifies me, but reading about something that could conceivably happen very close to where I live made it even scarier.

The Road by Cormac McCarthy. once you know about that basement scene you can never unknow it :S

Stephen King’s It