This novel doesn't have much to do with the first one in the Gateways series (at least, nothing that is apparent now). If you haven't read the New Earth series, especially "Challenger", you'll be lost. And if you like to read about Kirk and the rest of TOS crew, you'll be disappointed -- they're nowhere to be found. But I think this is a good thing.
One problem with Star Trek books is that the authors have to use characters, locations, and situations that are already familiar to us from hours of TV and movies. You miss the creativity of sci-fi epics where the writer shapes the whole universe (Dune, Heinlein, etc)
Which is why I enjoy the Challenger novels (and Peter David's Excalibur series) -- the authors are free to create totally new crews, and getting to know new characters is exciting.
For instance, in "Challenger", we saw a lot of self-doubt and second-guessing from Nick Keller. There's less of that in "Chainmail" -- he's more confident and more assertive. We see him growing as a Starfleet officer.
This book starts off fairly slowly, and early on there are some chapters written from the aliens' perspective where you have no idea what's going on -- a device I'm not fond of. However, by about halfway through the book all the threads of the story come together, and I couldn't put it down. It's got all the elements that make TOS my favorite Star Trek franchise: a clash of cultures, a mysterious, ancient and powerful civilization, eccentric characters, and a minimum of techno-babble. The characters rely on their brains and diplomacy rather than on technology.
Unfortunately, the novel is just a first part, with a "to be continued" at the end. But at least the book was good enough that I'm anxiously awaiting the continuation.