Well, the title says it all really. Even without visuals to make things more vivid, words written down on paper can sometimes freak me out. The creepiest book I've ever read was 172 Hours on the Moon, by Johan Harstad. There are many references to creepypastas/urban legends, and the main plot is quite creepy too. It didn't scare the ---- out of me, but I never really do read horror stories.
So, post the scariest book you've ever read, and why you think it's scary/creepy.