Here are some suggestions, some King and some not:
Gerald's Game, by Stephen King (though many people didn't like this for some reason, I found it to be a very solid book, very unsettling, and at times extremely scary. It is one of a select few books out there that stays with me and is still with me--some of that imagery is impossible to put out of my mind, and while reading the book I found myself somewhat jumpy and genuinely scared, and that is an amazing feeling while reading a book! You never really know, since being scared is a personal thing and what scares me might not scare you, but this one was...awesome)
The Ruins, by Scott Smith (though you may find it more disturbing than actually scary, I found it to be a fine mixture of both psychological terror and blatantly disturbing. Smiths' writing is fluid genius, sometimes like some sort of dark poetry, and the story is one of the best modern horror stories out there)
Ghost Story, by Peter Straub (this is one of those books I was talking about, the kind that stays with me. The terror is a sort of subtle horror that just grows and grows until, at times, you feel like you're living it. Written brilliantly with a very dark, gothic, and genius take on that of a traditional ghost story, it's a true horror classic)
The Tommyknockers, by Stephen King (to me, for some reason, this was one of his scariest books, partly because of how bizarre it is. From the beginning it sets this strange, dark, just wrong atmosphere that put me on edge--i must suggest reading this book, or any I suggest, primarily at night for the best effect--and for some reason scared me, a lot. This one had a moment or two where I actually set the book down in fear, for the sake of my being able to sleep. And THAT is a gift unlike any horror book could give)
IT, by Stephen King (depending on what scares you, this could be the scariest book you ever read, so long as you read it with an open mind, and without analyzing it, and let yourself be immersed into the reality of the story. The characters are the deepest and most real--most human--I think i've ever come across in a book, and therefore the entire book felt hauntingly real. I regard this book as an experience, not a book, and it is the best that I have ever read, my favorite of all time. Gory, psychological, unsettling, disgusting, disturbing, frightening and beautiful, this book is unlike any other out there)
Hell House, by Richard Matheson (this is, in my opinion, the best haunted house novel ever written. Believable not only in its storytelling but also on scientific and parapsychological levels, I regard this as one of the most disturbing books I have ever read, except it is disturbing on levels you wouldn't expect. It uses the power of subtle suggestion but also blatancy in its scares, and generates a very unsettling atmosphere perfect for the book that it is, and Matheson's writing is perfectly concise yet brilliant at the same time. If you want one like it, read the inspiration for all haunted house novels: "The Haunting of Hill House," by Shirley Jackson)
Hopefully some of these appeal to you!