I think my final opinion of Nona the Ninth is extremely biased by finding out at the end that it wasn't the end of the trilogy as expected. About halfway through, I was still kinda just enjoying the ride, watching some pieces of info come together, picking up some additional background. I wasn't super into Nona as a character, but whatever. Then I got to the end, and the Big Twist, and suddenly the entire book felt like a waste.
I thoroughly enjoyed both the Gideon and Harrow books. I appreciated how Harrow was just batshit insane and a very unreliable narrator in her book, and how different it was from the first. It was a pretty chaotic ride. I think if Nona had actually wrapped up the storyline, it would have made the meandering and needless code naming of prior characters worth it. Instead, it was just very unsatisfying to me. It felt like filler.
Thoughts on Nona the Ninth, spoilers for the Locked Tomb series
I do still very much enjoy Tamsyn Muir's writing style. I would not call this a "bad" book. As I said, I am likely biased because I expected this book to wrap up the trilogy. All the same, I will likely not read the next book until there are plenty of reviews out so I can find out if there will be yet another bait and switch.
Harrowhark Nonagesimus is still one of the best names I've read anywhere.