So, adult horror. Well, I like quite a few of these but I will do my best to narrow it down to 4 or 5.
The Haunting of Hill House by Shirley Jackson

What's makes this book very effective is the use of language and atmosphere. Jackson creates a believable and psychologically chilling world where at times you are not totally sure if things are really happening or if the people in the story are crazy. Oh, and the commentary of sexual perversion and repression alone make this one worth a read. Subtle, dark, and tense this story has been copied multiple times to poor effect. Jackson did it best, one of the earliest and best haunted house stories out there.

On a crisp evening in October a strange carnival rides into town and two boys' lives will never be the same. Both the boys and one father realize that something is not quite right about this particular circus show. But as they investigate will they be able to resist both the seductive pull of their own desires and the terrible needs of the Shadow Show?
The carousel scenes have stuck with me all these years and the variety of bone-chilling evil in this carnival is almost more than I can stand on a late October night. It is not a slasher or even outright evil that grabs me most; it is the insidious call to normal human needs and desires and how hard we will fight to fulfill them even to our own detriment that really makes this a memorable story.

Strangely this book takes place in mid-February, not a time typically associated with horror, but it likely should be. For when is life at its most boring and routine than the month of February? Also, this might not strictly be an adult book since it has been published as teen and children's as well. However, for me, some of the themes place it in a more adult sphere.
Harvey is a typical 10-year-old boy in a typical Midwestern town, until he meets a peculiar man with a huge grin who tells him of a house where every day is a holiday and children play all year round. Great, right? But what happens when you go to often or stay too long in a place of non-stop fun? And what is the price you will have to pay?
The Hunger by Whitley Strieber

A weird and sad atmosphere full of 80s references and cold beauty as well as questionable science make this one a fun read. Plus, I used to study primates, so the parts about the testing on apes and how closely they resemble us also pulled me in. And since it is a vampire story, there has to be some elements of sexuality and death. The most chilling part for me was what happens to her companions. That makes Miriam a true vampire villain to me.
Oh, and of course, there is a movie. With David Bowie....So get on that!

It is really hard to pick just one King book since I pretty much love them all. But if I must, The Shining was probably one that scared me the most the first time that I read it. Of course, I was working second shift at a hotel by myself at the time, so that could be one reason I found it so scary. Or maybe it was the real conflicts of alcoholism, parenthood, and adult responsibility (which I was just beginning to face at 20) that made this one stick.
In case you haven't read it, the Torrence family moves into The Overlook Hotel over winter to take care of the hotel while it is closed and so the father, Jack, can finish writing his Great American Novel. But they are not really alone, since the hotel hosts a wide variety of evil spooks that never really left and that hope to keep the Torrences with them--forever.
Well, those are some of my favorite adult creepy reads. What are yours?