An excellent tradition (started by Neil Gaiman) that has been taking place in October over the past couple of years: give the ones you care about something horrific to read for Halloween!
Here are my recommendations:
Pet Sematary -- To begin with the standard fair, this is one of Stephen King's best. Also try The Dead Zone, It, and The Shining. I would also add 'The Tommyknockers,' a highly underrated King novel.
I Am Legend -- Besides Bram Stoker's Dracula, this is the only other story with vampires I would recommend. Please don't bother with the movies; I'm sure Richard Matheson was none to pleased with any of them. The twist at the end of this one elevates I am Legend to literary heights.
Shadowland -- I considered Ghost Story, or even Floating Dragon, but Shadowland is a superior effort on Straub's part, in my opinion. A very interesting read. Very creepy.
American Psycho -- A gruesome comment on the wealthy American consumerist culture. Some of the most vivid descriptions of violence and sexual depravity ever in print! Highly recommended. ;)
V. -- There is horror in this challenging book, and an air of the macabre. Easier than Gravity's Rainbow, but more challenging than The Crying of Lot 49. Thomas Pynchon's V. has certain implied supernatural elements that make it acceptable for any list of horror literature.
The Collector -- John Fowles's first novel, and it is disturbing. If you cut through all the literary commenting on the English class system, you get a story about a deeply disturbed man holding a young liberal woman captive in his basement, with unpleasant results.
House of Leaves -- A postmodernist masterpiece of twisting and turning...literally! Prepare to read this one sideways and upsidedown. There are some very frightening parts to this spiral into the void...
The Turn of the Screw -- A classic in the "ghost story" genre by Henry James. Or is it? Ambiguity of character and story done with such precise mastery, it leaves one in a state of awe, and totally creeped out.
The Road -- Cormac McCarthy is depressing. That's a fact. And The Road is a trudge through hopelessness, but done with such vivid clarity and strength of spirit. Wonderful!
The October Country -- Ray Bradbury wrote some truly horrific stories in his early years and many of his best are collected in The October Country. "The Next in Line" is particularly good, as are such classics as "The Jar," "The Small Assassin," and I would add "The Emissary."
Books of Blood -- Clive Barker's short stories are fantastic and gruesome and well worth your time. Very good stuff.
From Hell -- To add a graphic novel to this list--Alan Moore. This one is excellent, stick with it to the end and you won't be disappointed.