..the Cthulhu stories by H.P. Lovecraft. I've read Shadow Out of Time, The Call of Cthulhu, and the first part of Whisperer in the Darkness, and loved them.
I'm not much of one for the horror genre. There's a ton of junk in it. However, Lovecraft is a master of the genre; he can evoke a shiver with a single phrase, and the end of his stories make me want to run and hide under the nearest couch. He manages to bring forth the horror of the utterly Other, the terror of chaos, of the "old gods" coming back to the earth. Some of the most disturbing parts of his classic, The Call of Cthulhu, are the sections in which the architecture of the ancient city is described as "all wrong," planes that are neither vertical nor horizontal, nor quite diagonal. Even the words, "Ph-nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn," translated as "In his house at R'lyeh dead Cthulhu waits dreaming", seem full of menace, even before the translation.
So...yeah. Creepy as all get-out, and great stuff. Hmmm...my birthday is coming up in July....a book of Lovecraft stories would not go amiss....... :)