Kind of an odd duck. The earliest description made it sound like a Kirk book. But while he certainly features in the novel, this is really a Sulu book. I wasn't sure of the pacing until they got to the second planet and found the second city. I was thinking it was weird that the first planet incident took up a third of the book and then seemingly moved on.
By the time they get to the second planet, I was getting into it. However, I was a bit confused because it seemed that the book had been shifting back to Kirk's future.
Anyway, it was a little distracting about how obvious the guest character and guest Starfleet vessel were red shirts here. I agree that the hints at Trinh's past seemed to be left dangling. If it was supposed to connect back to her first husband, I didn't really get the connection. And Sulu's issue with Kirk was a little out of left field and obviously temporary as we see the two serving together again later.
But overall I liked the mystery and the last minute revelation about the two alien species was pretty cool. That's most of what I've wanted to see from TOS books: connections with other series. It was a bit last minute however. I wish it had come up earlier.
The revelation did raise some questions though:
Also, When are we going to see Sulu's eventual family?