Concluded reading just now.
I picked up the book because it featured Mestral but caught captivated by the book's plot, and not just for Mestral's inclusion.
Immediately I warmed up to Wainwright and Allison, as well as other characters like Gejalik and the Aegis agents. Imho, it was the best TOS outing this year but then, I'm easy to please.
The return of the Tandarans was just another extremely pleasing example of tying TOS in with the rest of Trek
continuity. Here you've got a standalone adventure that still part of the continuity tapestry. Fabulous!