I'll start the ball rolling with
The Mysterious Mr. Quin by Agatha Christie.
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Spoiler:
From Amazon:
The Queen of Mystery has come to Harper Collins! Agatha Christie, the acknowledged mistress of suspense—creator of indomitable sleuth Miss Marple, meticulous Belgian detective Hercule Poirot, and so many other unforgettable characters—brings her entire oeuvre of ingenious whodunits, locked room mysteries, and perplexing puzzles to William Morrow Paperbacks. The inimitable Christie intrigues, surprises, and delights once again with The Mysterious Mr. Quin—a riveting collection of short stories centered around the enigmatic Harley Quin, whose unpredictable comings and goings are usually a good indication that something is about to happen…and rarely for the best.
Excerpt from Wikipedia:
...Each chapter or story involves a separate mystery that is solved through the interaction between the characters of Mr Satterthwaite, a socialite, and the eponymous Mr Quin who appears almost magically at the most opportune moments and disappears just as mysteriously. Satterthwaite is a small, observant man who is able to wrap up each mystery through the careful prodding and apposite questions of Quin, who serves as a catalyst every time the men meet.
In Agatha Christie's Autobiography, she claims that Quin and Satterthwaite became two of her favourite characters....