Silence of the Lambs by Thomas Harris. It was awesome. And every horror book I have read since has tried to scare me that much, and so far none have.
This October my scary book of the month is Those Across the River. It's the first novel by Christopher Buelman. And though not as scary as Silence of the Lambs, I am enjoying it nearly as much. It is a carefully woven story that brings in history, atmosphere and interesting characters together in a chillingly satisfying novel. In 1935, Frank Nichols moves his wife Dora to Georgia to research a book on his Grandfather who was the last slaveholder to let go of his slaves after the emancipation. Upon arriving Frank is warned not to cross the river, but that is where the plantation resided.
I don't want to give too much away. But if you like a scary story mixed with some interesting history, this is a great read.