Stevenson

Dr. Henry Jekyll is a respected London physician with a theory: that every person is really two people, one decent and one depraved, locked together against their will. He brews a potion to separate them cleanly - and Mr. Edward Hyde is born, small, violent, and utterly without conscience. What starts as a controlled experiment in privacy becomes something Jekyll can no longer switch off, as Hyde grows stronger with every use and starts appearing uninvited.
The novella that gave the world "Jekyll and Hyde" as shorthand for anyone with a hidden dark side - still one of the sharpest, tightest psychological horror stories ever written, and still capable of unsettling a reader who already knows the twist.