John Rhode 
Robthorne Mystery 

The Robthorne Mystery, first published in 1934, is part of the series of mysteries featuring private detective Dr. Priestley. Author John Rhode, a pen name of Cecil Street (1884-1964), was a prolific writer of mostly detective novels, publishing more than 140 books between 1924 and 1961.When one of the Robthorne twins commits suicide, there is a question over which one it is and whether it was, in fact, suicide or murder. From the dustjacket: Dr. Priestley, well-known crime investigator, is called in to solve the mysterious death of Mr. Robthorne, who has been found shot in the greenhouse of his country estate. A chain of damning evidence that Dr. Priestley pieces so successfully together forms one of the finest examples of crime detection that Mr. John Rhode has yet produced.