Robert Louis Stevenson 
The Strange Case of Dr. Jekyll and Mr. Hyde [EPUB ebook] 

It is about a London lawyer named Gabriel John Utterson who investigates strange occurrences between his old friend, Dr. Henry Jekyll, and the evil Edward Hyde.The work is commonly associated with the rare mental condition often called ‘split personality’, referred to in psychiatry as dissociative identity disorder, where within the same body there exists more than one distinct personality. In this case, there are two personalities within Dr Jekyll, one apparently good and the other evil. The novella’s impact is such that it has become a part of the language, with the very phrase ‘Jekyll and Hyde’ coming to mean a person who is vastly different in moral character from one situation to the next.