Confounding Oaths.

"Confounding Oaths" by Alexis Hall is the second book in the Mortal Follies series and follows John Caesar, a nobleman in 1815 London, as he becomes infatuated with Captain Orestes James. When one of Caesar's sisters makes a deal with a fairy, Caesar and James must work together to unravel the magical consequences. The story explores the strictures of Regency England and includes discussions of race, as both protagonists are mixed-race. The book can be enjoyed as a standalone and features cameos from characters in the first book. [Extracted from the article]