I have a suspicion that The Prisoner of Zenda is one of those books where every adaptation/retelling is better than the original, and the only reason I can't say that I'm certain of it is that I've never seen a straight-up film adaptation of it all the way through. The first introduction I can remember to the basic plot was a two-part episode of Get Smart, and although I recognized it in several other books that I'd read in the decades since then I didn't bother to read the original until last year when Charles announced that her version was forthcoming. I found it to be decent pulp nonsense and that's about it. Now, Charles's take on the story will bear rereading. If you're familiar with the original, you won't have much trouble seeing how Detchard's side of the story fits into it. I could totally buy Rudolf V as a drunken debaucher and worse, given our brief introduction to him at the beginning of Prisoner. And that Flavia and Antoinette have schemes of their own makes more sense than anything Hope came up with to explain their motivations. I would say that this is the least genre-romance of Charles's books: Rupert of Hentzau is a sexy fellow and was so even in the original, and much sex is had between him and Detchard once the latter discards Black Michael's orders not to fraternize in that way, but I never felt like the book was driven by the question of how the two of them would arrive at a HEA or even a HFN. It's a gay swashbuckler more than anything else.
This book also features a pun so glorious that, even though I'd read about it on the KJ Charles FB group, I still had to stop for a minute when I actually read it and think "I can't believe she actually did that to me."
Overall: A