Death in Harley Street

''Death in Harley Street'' (1946) is widely celebrated by genre historians and critics as the author's ultimate masterpiece of Golden Age detective fiction. Dr. Priestley is a retired, eccentric mathematics professor who operates as an armchair forensics specialist. He leaves the physical footwork to Scotland Yard's Inspector Jimmy Waghorn while he solves cases purely through logical deduction.
The premise centers on the sudden, violent death of an eminent medical specialist, Dr. Richard Ernest Knapp Mawsley, who is found dead on the floor of his locked consulting office on Harley Street. The forensic evidence shows he died from a self-administered injection of a fatal dose of strychnine. However, the case presents an impossible puzzle. There was no suicide note. Dr. Mawsley was an exceptionally careful, happy man who had just gleefully received word of a large inheritance. No signs of intrusion.
The room was completely secured, and the clinic staff could account for everyone's exact whereabouts. No apparent accident. Dr. Mawsley was far too methodical to mistake a strychnine bottle for standard medicine. This tale is highly praised for introducing what critics called the "Fourth Dimension in crime"-proving a death occurred by a method outside of standard categorization.