There are tons of scary books that work for teens - including some adult titles. I for sure read Carrie and The Shining (both by Stephen King, as if I truly need to tell you that) when I was a teen (junior or senior in high school) and they were perfect for me at that age. (I am, in fact, giving these books to my girls for All Hallow's Read this year.) One Stephen King book that works for younger teens is The Girl Who Loved Tom Gordon, in which a 9-year old girl named Trisha gets lost in the woods, straying far from the trail. She spends days on her own trying to make her way back to civilization, eventually succeeding, aided by her determination and her fondness for pitcher Tom Gordon.
Devoured by Amanda Marrone is one of the scariest YA books M has ever read, which is why it makes this list. Not only is the main character enlisted to dress up as Snow White for a particular attraction, but she is also subjected to the owners' hall of mirrors, which includes a particularly nasty sort of magic mirror. There are ghosts in this book as well, but the ghosts are actually far less scary than some of the actual humans with whom Megan must deal.
Killing Britney by Sean Olin is creepy/scary indeed. The main character, Britney, proves to be completely unreliable in this book, which I found so disturbing I didn't want to keep it in the house, so I donated it to the library. The body count piles up and it appears that someone is trying to kill Britney, but most readers will not see the twist coming in this one.
In stores now, Zombies v. Unicorns, edited by Holly Black and Justine Larbalestier, is one heck of an anthology, combining funny with wistful and including a bit of the scary. My favorite story in the book might be Margo Lanagan's A Thousand Flowers, and it's decidedly a twisty/twisted tale that will horrify you for sure.