Ok, not a book, but this seems to be the most appropriate place for this.

Who here reads/watches things in the Slender Man mythos? For those that don't know, Slender Man is creepy and scary. If you're still interested, have a look at Marble Hornets, that's what got me into the mythos.

The basic theme in Slender Man stories is that the protagonist is being stalked by the Slender Man, a superhuman entity that steals away children, drives people insane with fear, and causes memory loss. He is utterly alien - which is why I like him, he appeals to the Lovecraftian cosmic horror fan in me

In fact, feeling that the Slender Man mythos doesn't already have enough contributing blogs (ha, I jest, it has many) I have started a slenderblog myself Is it Slender Man?, written from the perspective of a mentally disturbed Slender Man "veteran".

Maybe I can use this for ideas in NaNoWriMo this year, too…

Anyway, I love cosmic horror, Slender Man is comic horror, and you should also investigate/enjoy