John Rhode 
Mysterious Suspect 

The Mysterious Suspect, first published in 1953 (and also known by the title By Registered Post), 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.In The Mysterious Suspect, wealthy industrialist Peter Horningtoft is found dead in his study after apparently drinking poison from a bottle sent to him as a rheumatism treatment. Jimmy Waghorn is called in and blunders through the case initially until assisted by Dr Priestley. A second murder, disguised as a suicide, re-ignites the investigation.