A Question of Hats — Clara Benson

When a mysterious parcel containing a hat is delivered by mistake to Angela Marchmont she thinks little of it, except to note that she wouldn't be seen dead in the thing. But if she doesn't want it, there are plenty of other people who do -- and who are prepared to take it from her by force if necessary. It soon turns out there's more than just a hat at stake, and now it's up to Angela to discover what's been going on -- before someone else gets there first!

A Question of Hats is a short story of seven chapters, which takes place between the events of The Incident at Fives Castle and The Imbroglio at the Villa Pozzi. There are no spoilers for the rest of the series!