Misery

 Not a scary book in the traditional sense, but definitely one that I found very unsettling and the tension runs high reading it. I don't think anyone writes crazy, quite like King does. He's written a lot of nasty bad guys, supernatural boogeymen and antagonists of other sorts in his time, but Annie is definitely up there as one of the most memorable and compelling to me. A bit over the top at times mind you, but overall she's what makes the book work.

If you love psychological horror then this is one I'd definitely recommend.