I'm going to nominate Robert Louis Stevenson's classic story "The Strange Case of Dr Jekyll and Mr Hyde", which is what we'd call a "novella" today.
In case anyone isn't familiar with the plot, Dr. Jekyll is a kindly doctor who is experimenting with "mind altering" drugs. One of these drugs accidentally turns him in a homicidal maniac, who calls himself Edward Hyde, and whose personality gradually takes over that of Dr. Jekyll, until he can no longer control his transformations. A story of a man's decent into madness. Still as gripping today as when it was written in 1886.
Last edited by HarryT; 09-21-2010 at 03:57 AM.