The Great God Pan
If I could take any story (other than my own) and turn it into a movie it would be Arthur Machen's 1890 novella The Great God Pan. You want to read what Stephen King called "one of the best horror stories ever written. Maybe the best in the English language"? The story about which Lovecraft said: "No one could begin to describe the cumulative suspense and ultimate horror with which every paragraph abounds"?
Of course you do. Go here.