E.Phillips Oppenheim 
Double Four 

The book The Double Four opens with Peter Ruff who chooses to leave the criminal underworld for the more lucrative career of a private investigator. Peter is the ideal investigator because of his connections to the underworld and his ability to appear as somebody else. Mr. Ruff is incredibly prosperous in his new job, and Violet Brown, his trustworthy and beautiful assistant, helps him out tremendously. He is seen and accepted as a leader by a large group of criminals who have decided to use their skills for good as his reputation for solving crimes increases. However, his fame makes many people hate him, including the new spouse of his ex-fiancee and a prominent German spy. This wonderful collection of short stories by a mystery author, whose brilliance, sadly, has been forgotten by many people.