Which 1930 Christie short-story collection features Harley Quin?

The 1930 Christie short-story collection featuring Harley Quin is The Mysterious Mr Quin.

Published in the United Kingdom by William Collins & Sons on 14 April 1930, the book contains linked mysteries involving the enigmatic Mr Quin and the elderly socialite Mr Satterthwaite. Quin appears unexpectedly, often at moments when a hidden truth or unresolved tragedy is ready to surface.

Harley Quin is not a conventional detective like Hercule Poirot. He usually acts as a catalyst, guiding Satterthwaite toward deductions rather than solving cases by ordinary investigation. Christie drew on the traditional Harlequin figure, reflected in Quin’s colourful imagery, mask-like appearance and elusive entrances and exits.

A common mix-up is to confuse this collection with The Harlequin Tea Set, a later collection that also includes Quin stories. The title is also sometimes written with a full stop—The Mysterious Mr. Quin—but the stored answer uses the standard title form.