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

The classic psychological mystery. According to Wikipedia: 'Robert Louis Balfour Stevenson (13 November 1850–3 December 1894), was a Scottish novelist, poet, essayist and travel writer. He was the man who 'seemed to pick the right word up on the point of his pen, like a man playing spillikins’, as G. K. Chesterton put it.’