The Clue by Carolyn Wells audiobook.
Once Carolyn Wells began, or re-invented her writing career, 'The Clue' was her initial book which strayed from children's writings into mysteries and detective stories. It is also when we are introduced to her most famous of detectives, Fleming Stone.
On the eve of her wedding day, Madeleine Van Norman, a beautiful young lady who is soon to come into her family fortune is found dead, apparently stabbed with an ominous blood-stained letter opener found nearby. There is nobody within the household who is not considered a suspect by the police, but how could a killer have slipped through the doors of Madeleine’s locked bedroom? It must have been suicide, as a note was found lying on a table near her body. Or was it? An intriguing mystery ensues which hinges on the discovery of a single, all-important clue.