The Strange Case of Dr. Jekyll and Mr. Hyde; by Robert Louis Stevenson.

In many ways this work was pioneering, in exploring the concept of drug related personality change and writing about it, depicting the effect of drugs on psyche to the extent that a person under effect of a drug can chane completely in personality and behaviour; however that aspect of the work was pushed aside, buried, and got lost in the sensation of dual personalities of the title and the rest of it, the horror, the murder without mystery and so on.

In a way it might be about the writer himself - when you read his poetry you cannot imagine him creating a Hyde, and vice versa. (Don't tell me they are separate people, that would make sense!)