You can't go wrong with Dean Koontz, and I think if you're going to go with supernatural for Halloween as opposed to one of his serial killer books, I'd definitely pick "Phantoms". Or "Shutter Island" by Dennis Lehane, which gave me nightmares.
For realism (sort of) I'd go with "The Perfect Husband" by Lisa Gardner. Very disturbing.
Dean Koontz ~ Watchers (my absolute favorite of his), The Servants of Twilight, Midnight. Basically his older stuff.
Stephen King ~ Pet Sematary, Misery, It.
I think I'll pick up some of these and re-read them once I'm able to get focused on my reading.
~*~*~ Kim ~*~*~
"In the Forest of Harm" by Sallie Bissell is probably the most horrific book I've read. Three female friends go hiking & camping and meet up with the worst possible evil.
My favourite scary book is Ring by Koji Suzuki. I saw the original Japanese film after reading the book and I thought it was rubbish - so I wont even think of watching the American re make. The book is really creepy and I was on edge for weeks after finishing it. Also, Salem's Lot is another creep fest! Gave me nightmares - literally.