The Strange Case of Dr Jekyll and Mr Hyde1 is a novella written by the Scottish author Robert Louis Stevenson first published in 1886. It is about a London lawyer who investigates strange occurrences between his old friend, Dr Henry Jekyll2, and the misanthropic man Edward Hyde. The work is known for its vivid portrayal of the psychopathology of a split personality, in mainstream culture the very phrase "Jekyll and Hyde" has come to signify wild or polar behaviour.
http://en.wikipedia.org/wiki/Jekyll_and_Hyde
by the way, Robert Louis Stevenson is the author of Treasure Island - one of the greatest classics I have ever read