*** Irene's Last Waltz, Carole Nelson Douglas, 1994
Douglas does a great job of weaving canonical story lines into a new story, and this one is no exception. There are a lot of things happening on a lot of levels with several nice appearances of a certain personage.
One of those plot lines involves Charles Frederick Worth, the legendary designer. As a result, I resorted to a bit of skimming over the copious descriptions of fashion. YMMV.
This book was reissued as Another Scandal in Bohemia in 2003.