I think I've stated this before on this very blog, but it bears repeating: there are only a scant handful of horror books that have actually succeeded in terrifying me. Dan Simmons "Song Of Kali" is probably the most powerful, which is why I've saved it for last.
"Song of Kali" is the story of Robert, an American journalist, who is sent to Calcutta, India to investigate a deceased poet, who might not be dead after all. His descent into the dark world of Kali-worshippers and the living dead lead up to one of the biggest and most brutal punches to the gut I've ever experienced with a book. It's truly horrifying.
Not recommended to the squeamish.