Scariest book I've ever read

Despite the fact that I love horror films, I don't often read horror books, so I had to think about this topic from Long and Short Reviews. Somehow the creep factor that I enjoy in a film doesn't seem to carry over into books for me. I don't mind the occasional Stephen King, such as Carrie, but I didn't find it especially scary.

So I ended up choosing The Woman in Black.

The Woman in Black has been adapted into films and a play, both of which are quite a different animal from the book. Certainly the most recent film, while scary, had very little in common with the book apart from the woman and the rocking chair. The book, however, while understated, does have a certain poignance that grips you, making you want a happy ending even though you know there won't be one.

So what's the scariest book you've ever read?