I don't read much horror, and I'm not easily scared by books, or even films, for that matter.
However, I read The Amityville Horror a few years ago and I don't mind admitting, it cared the bejesus out of me. I still think about parts of it even now - the flies, the ornamental lion and the red room - and it really does haunt me.
At age 13 I had the misfortune of becoming an Edgar Allen Poe junky, in the vernacular. Some were just troubling, rather like the burp one has with a bad hot dog some 3 hours after consuming it. Other of Poe's tales have a way of staying with you to re-emerge years later with the same shiver up the spine.
I thought I had put that all behind me until re-reading an anthology of Poe's best. I handled the Cask of Amantilado well and even Murders of the Rue Morgue... but then I made the mistake of reading The Premature Burial late on evening when I was alone in the house.
Poe has the eerie ability to depict unltimate horrors without delving into todays genre' of blood and guts spilled willy-nilly by pubescent teenagers chased by a guy in a bad suit and even worse hair wielding a chainsaw. Naw... none of that; but imagine yourself in a 3 foot wide wodden box with tons of dirt on all sides muffling any sound or movement you could make.... it'd probably take you a week to die, leaving deep scratch marks on the coffin lid, just as Poe so eloquently describes... Don't read this alone with dim lights!
the Shining by stephen King. The intense build up of paranoia and eventual madness was palpable at times. it was a dreadful shame that the film which was excellent in some parts,chose to play the terrifying johnny/wendy bathroom scene for laughs. if you were reading the book,you would definetely not be laughing at that point
I don't know about frightening but Irvine Welsh's 'Filth' is absolutely horrifying... The imagery is unfortunately, graphic and stomach turning. Although admittedly it's not scary.
I also have mental scarring from reading The Shining in my 20s. I was so scared of it I hid it in a drawer but then had to get it back out to finish the book because I needed to know how it ended. The film was a disappointment.
Anything by MR James is also pretty scary - I literally jumped out of my chair reading some of his stories.
Recently the most disturbing thing I've read is Under the Skin by Michel Faber. It defies classification - a mix of sci-fi and horror. It is probably the scariest thing I have ever read - deeply deeply terrifying, but beautifully written.