The problem with being a truly gifted storyteller is that you get away with so much. I got this out of the library because I'd asked for and gotten an ARC of the sequel without realizing that it was a sequel & I didn't want to go into it clueless. And *heaves sigh* I had much the same experience with it as with the companion Magic in Manhattan series -- namely, I alternated between gulping it down and tearing my hair out, frustrated not even so much by the errors/missteps themselves as by how easily they could have been fixed.
A pub has a Scottish busboy named Calum. Which should be "Callum." And, sure, names have variant forms, but ... maybe don't distract readers with an oddly spelled name for someone mentioned only in passing, with no role in the story?
Savile Row, not Saville Row.
More than once, Wesley speaks of shell shock among his soldiers who were kept prisoner "behind enemy lines." Well, first of all, where else would they have been kept prisoner? But also (1) prisoners weren't treated especially badly; (2) prisoners were out of combat, which was a lot safer than being in, and (3) LOL at being taken prisoner as a standout cause of shell shock when conditions in the trenches were a freaking horror show, between the mud, the rotting bodies, the rats and lice, and of course the constant threat of hideous death. Wounded men were drowning in shell craters, for goodness' sake.
Wesley tells Sebastian, "You pack a far harder punch than a shot of whiskey, which is all the anesthetic most soldiers got.” Um ... no? Stretcher bearers were supplied with morphia and so were casualty clearing stations and hospitals. Also, rum was the form of alcohol supplied to the troops. (In fact, I'm reading an ARC right now [The Life of a Medical Officer in World War I] that mentions the rum ration.)
And so many off notes in the register of people's speech. "That's not a thing." This wasn't even a usage 50 years ago, much less in the 1920s. Wesley calls Sebastian an "absolute nob." A nob, per Merriam-Webster, is someone in a superior position in life -- a synonym would be "toff." Wesley presumably means "knob," UK slang for "penis." I have no idea when it came into common use, so it could have been WWI slang that Wesley picked up in the British army. Is it a little low-rent for a viscount? KJ Charles probably knows. But Wesley's later "I'd be proper vexed" sets off all my speech-register alarms.
I said that mistakes and off notes like these could have been easily fixed, but I don't know. You need to read a lot of documents (novels, collections of letters) from the relevant period, and/or read a fair amount of history, and/or have a good nose (or a copy editor with a good nose) for what needs checking. And, as I said, Allie Therin mostly gets away with it because she's so damn good at telling a story. If she had been a little more scrupulous in her research, this would've been an easy 4 stars.