I think that if you like Reign or the Luxe series, you'll probably really enjoy this book: it has a loose historical flavor while actually being a soapy drama about rich modern young people. And that's okay! But I was expecting it to have more substance.

The premise of the alternate universe/alt-history is very interesting - England won the Hundred Years' War, creating the British Empire several centuries early and holding onto it into the twentieth century, and magic is real. Unfortunately, the book just didn't delve into that (beyond the issue of having a Valois family in the shadows) and it's all kind of superficial, an excuse to avoid having the actual tangled politics of Europe in the 1900s/1910s and a way to make the crown extremely important. It just felt like there was so much scope for worldbuilding there that was just left on the ground.

I'm going to be brief on this point because I have to accept that going into a fantasy/historical/romantic YA novel was the big signal that we weren't going to have actual historical attitudes, but it was annoying to me that the characters were just making out and having sex all over the place. It felt particularly problematic wrt Isabelle, the last Valois, because she seemed to be written as the sexist stereotype, "the damaged mean girl who has too much sex". (Edit: And this bothered me but not too much, because I assumed there was a sequel where she'd at least get a happy ending à la Esther Denham in Sanditon, but in the absence of a sequel it's really, really problematic.)

To be honest, I was happy with the plot being wholly centered around courtship and romantic drama. I like marriage novels! But then the last few chapters suddenly packed in all of this barely- or not-foreshadowed magical danger, and I was not very impressed with it. I think we probably did need to have magical danger given the magic of the setup, but it was very rushed.