The Road by Cormac McCarthy
There's something about post-apocalyptic fiction that appeals to a ton of people, and Cormac McCarthy's The Road is one of the biggest novels of the genre. It gained an even bigger audience because of the movie, but is a masterpiece in its own right. McCarthy's writing is perfectly stark and depressing, which perfectly fits the mood of a world ravaged by unknown disaster.
Scare Factor: 3 - Not the kind of book that relies on jumpy scares, but is more scary because it is so dark. Definitely has a lot more violence than some people will be comfortable with.