This is 
a question that a BookEnds asked: http://www.nytimes.com/2014/10/26/books/review/whats-the-most-terrifying-book-youve-ever-read.html
The writers chose books for various reasons, including how real the book felt.
He who controls the past controls the future. He who controls the present controls the past.
For me, the scariest book I ever read is Nineteen Eighty-Four. It’s dystopian, not horror, but the way they changed the newspapers after publication? Yeah. Not taking it back, not apologizing for a print error, but changing it altogether.
That kind of thing wasn’t possible when it was written, it wasn’t possible in 1984, but it is possible now. And that’s scary.
I love ebooks and reading online in general, but that kind of thing is possible and it scares me. It really does.
Story Treasury 
I never thought of 1984 as scary. But I guess you’re right.
No horror books in reading list ?
I read my fist horror book a couple months ago! So no. I don’t read enough of them.
Ha ha. I guessed so. I am myself afraid of horror as a genre.
A part of me is starting to suspect some (a lot?) of horror would be at home in urban fantasy. So I should explore it…
Actually no. Real horror is more commonplace. I am not afraid of fantasy horror, but give me the horror in real life and you will have me at pins and needles.
Hmmmm, I don’t know what the most terrifying book I’ve read would be. Maybe Stephen King’s It, just because I was so young and impressionable when I read it.
(The changing of the newspapers, post-printing IS scary. But man, so is the fear of the rats in the box. Ugh.)
1984 horrified me when it came out. The ending and how it showed a government could essentially erased the personality of the main character stuck with me for a long time. Plus the rats.
I read a lot of horror, fantasy and realistic. I would have to think really hard about which disturbed me the most. Though, the most recent to make me shiver before sleep was House of Leaves by Mark Z. Danielewski.