I have to be honest: The mystery totally disappointed me. It was almost an afterthought in the story and the resolution had nothing to do with the story.
But the setting is interesting and unusual (London haute couture paired with the Russian émigré community) and the characters are nice and relatable. So it was enjoyable anyway.
Full review here https://theoldshelter.com/madame-koska-and-the-imperial-brooch-book-review/