Just completed this novel. I really enjoyed JJ Miller's last Discovery novel, "The Enterprise War" and rated that one excellent. For me this was a bit of a step down. I liked the travels through Troika space, seeing all 3 alien societies, completely alien in fact. One advantage of novels (an animated series) is it's a lot easier to depict completely alien species without have to worry about making them appear on screen. And Miller takes full advantage of that.
Another interesting element of this novel is it could have concurrently had up to 3 novel headings. Discovery of course (even though the Discovery itself is barely mentioned it is definitely in that series). Another would be Section 31, as it clearly involves Section 31 (though you can argue a different Section 31 from the novel series), and less so the Mirror Universe. Most of it does take place in the Prime Universe though.
And I liked getting a story with Emony Dax. We don't know much about Emony and it was nice to get a story involving that little known character for once. And Miller did a good job capturing some of Dax's personality (at times you could see a bit of Jadzia and Ezri--obviously meaning they were part of Dax's personality) and some of what would be unique to Emony.
We also got to see Finnegan (as seen on "Shore Leave"). His personality is mostly as I would imagine it. A troublemaker, and a bit of a rule-breaker, but more on the misdemeanor side. He's not an 'evil' or bad guy. He clearly was unhappy in a soldier role, esp. fighting a war he did not believe in. And he was horrified at what happened to the Farragut crew. So he's not an amoral character. He likes a good brawl, but he's not a killer if he can help it.
I'm not too sold on the idea that the cloud creature from "Obsession" is what it turned out to be in the novel. Miller kind of lost me there. And stage 3 of the book (Plundering) which took place in Casmarra seemed to drag a bit for me. I don't think it's any longer than any other section, but for whatever reason I found that part of the book a bit of a slog. The Dromax and Oast sections seemed to move with more swiftness. The ending was a bit chaotic as well, with changing alliances and circumstances. And Emperor Georgiou's motivations are a mystery, though I imagine that's by design.
I read "The Enterprise War" before seeing season 2, which I thought worked out well for me. This is also a pre-season 2 story, though in this case I don't feel I've missed anything by reading this novel after. It gives you some background into Georgio from the mirror universe, but it doesn't seem to directly feed into season 2 as much as "The Enterprise War" did.
Overall I'll give this one an above average. What I liked outweighed the things I didn't care for as much.