Verdict: even sillier than normal for this series. On the plus side, I did not hate either of the romantic protagonists, as is so often the case. Most of the leads from her other books wandered pointlessly through the story to remind me of how forgettable they were. The only ones I found memorably hateful were Vaughn and Mary, who had even less excuse for their cameo than some of the other characters'. No, actually the character who had the least reason to be in the story was Jane Austen, weakly shoved in there in what seemed like a blatant attempt to get Austenites to pick up this fluff. Don't: she is a minor and poorly depicted aside. Willig implies that she's read Austen's own correspondence but she certainly doesn't get the tone anywhere close.
I did remember Turnip from his stumbles across the scene in previous books. In this book he reminded me of Heyer's Freddy Standish. He seemed more oblivious than idiotic, although I did not buy that he would be unaware of the social stigma attaching to unmarried women being along with men. And I wasn't a hundred percent sold on the rapport between him and the much smarter and more educated Arabella. But they were sympathetic and rarely annoyed me, which is saying something for a romance.
And this really was a romance. There really was not enough spy action to balance it. Willig is always obviously more interested in her romance than her espionage, but in this case that plot was incredibly weak and implausible. All sorts of people stumbling around a girls' boarding school in the night? Messages via Christmas pudding? Come on. And the villain was so unbelievable.
At least I did not have to skim through the unfunny Bridget-Jonesish stumblings of Eloise, our usual flustered and insecure narrator. For whatever reason, the contemporary frame narrative is completely absent! Odd, but as I had quickly gotten tired of Eloise I did not mind. I liked the original concept that she was a graduate student discovering all these spy adventures in the course of her research, but her story quickly became way too chick-lit.
Overall, a quick and disposable read, painless as long as you don't care about authenticity of language and behavior in your historicals.