Trent's Last Case (The Woman in Black) - Edmund Clerihew Bentley

Trent's Last Case (US title The Woman in Black) is actually the first novel in which gentleman sleuth Philip Trent appears. The novel is a whodunit whose unique place in the history of detective fiction is because it is at the same time the first major send-up of that very genre: Not only does Trent fall in love with one of the primary suspects — usually considered a no-no — he also, after painstakingly collecting all the evidence, draws all the wrong conclusions. Convinced that he has tracked down the murderer of a business tycoon who was shot in his mansion, he is told by the real perpetrator over dinner what mistakes in the logical deduction of the solution of the crime he has made. On hearing what really happened, Trent vows that he will never again attempt to dabble in crime detection. (Summary from Wikipedia)