Novels, comics, manga, music, animation, anything that deals with the genre of horror is acceptable.
It's rare that I find a horror novel that's genuinely terrifying though. Some authors are really good at making my skin crawl, others give a good creep factor, but the written word just can't take the place of the sights from say, The Exorcist. I guess I just have trouble visualizing horrific images.
As for manga, Parasyte really disturbed me, not the images but the story... again with the "Your fiends and family are not who you think they are" plot. Also one of the shorts at the end of Gyo, I think it was titled Holes or something similar.
Dawn of the Dead, unfortunately, is mostly a gore-fest. Though I do have to admit that even my face turned green at a couple of moments during the film when I first saw it, and I have always been pretty well inoculated against gore from having grown up going hunting with my dad and helping to skin, clean, and quarter deer and hogs.