Only in the last twenty pages did I realize that this is an actual noir story; until then I had been complacent and dismissive in the belief that it was a Regency romance coyly pretending to be a noir. I am grateful to have been so fooled; it reminds me of the line that 'you can't con an honest mark'.
I don't know why it needed alternate history, though. Surely the actual politics were sufficiently full of maneuvering and interest? I don't see that a Queen Regent makes a tough female character more likely: I didn't blog Charlotte : Being a True Account of an Actress's Flamboyant Adventures in Eighteenth-Century London's Wild and Wicked Theatrical World, but its subject was quite as reckless as any fictional heroine. (Her failure as a fictional heroine would be excess and inconsistency.)
Find in a Library: Point of Honour or
Charlotte