John Rhode - Hendon's First Case (1935)

"Not content with a case of housebreaking, you want to tack a murder on to it."

Hendon's First Case is the Dr. Priestley novel which introduces Jimmy Waghorn, a Cambridge graduate with an unremarkable academic history and a lack of ambition who, after his father was ruined in a financial crash, decided to enroll in the new police college at Hendon. Eventually coming under the mentorship of series regular Inspector Hanslet, Jimmy is given the opportunity to distinguish himself when Hanslet receives news of a break-in. Upon arriving at the address, Jimmy learns that the break-in occurred at the workspace occupied by two research chemists, Harwood and Threlfall, and that they were dining together at a restaurant when the break-in occurred. Harwood, who returned to the laboratory alone after dinner, speculates that a rival might have entered  the premises to steal or impede their research. But there is a further complication in the investigation. Harwood appears to be suffering from food poisoning and Waghorn later receives the disconcerting news that Threlfall was admitted to a hospital shortly after dinner and died from ptomaine poisoning. The ptomaine is traced to their dinner at the restaurant and, while Hanslet concludes that the meal could not have been deliberately poisoned, Waghorn is not so sure that it is not a case of murder.

The appeal of Hendon's First Case, apart from the introduction of Waghorn, lies in the mystery of how Harwood and Threlfall were poisoned. However, the means is obviously and clumsily introduced in testimony, so I'd characterize this as a lukewarm Dr. Priestley novel. There is an interesting discussion on deciphering codes however.