Death by Water - Kerry Greenwood

P&O have approached Phryne with a mission, to investigate jewellery theft, apparently someone in first-class is committing the deed, she's asked to pose as a first class passenger (not that Phryne would countenance anything less these days) and use a lure to catch the thief.  Things start getting complicated and everyone is a possible suspect.

 

Poor Dot gets knocked out, but Phryne does her bit in panache and elegant clothes.

 

Twisty plot with lots of red herrings and plenty of moments where Phryne is magnificent.

 

This qualifies for Amateur Sleuth, Terrifying women and possibly locked room mystery, also probably country house mystery, I'm going to go with Terrifying women