Strange Case of Doctor Jekyll and Mr. Hyde by Robert Louis Stevenson
Robert Louis Stevenson was responsible for a couple of classics, and Doctor Jekyll and Mr. Hyde is one of them. It deals with the duality of man, the inner struggle between good and evil. It has not only been the basis for a ton of films, but is also cited as one of the key inspirations of the development of the Incredible Hulk for Marvel Comics.
Scare Factor: 2 - It was probably scarier at the time, but the horror is a bit dated now. Still, it has creepy moments and a great mood.