The Great God Pan, a short story by Arthur Machen about the intrusion into Edwardian London of a great evil, is one of the few genuinely disturbing pieces of fiction I’ve ever read. It’s disturbing in the way that a horror film can spook you, or even a personal trauma. The story sets on you only after you’ve finished it and are putting the pieces together.
I was heartened to see this appreciation of Machen over at The Paris Review. If you don’t have time to read Machen, but want to understand his importance to the modern literature of the uncanny (an ever more important mode of expression for our literary mainstream), then the piece is worth your time.