This book is set between the original 5-year mission and the Animated Series - Chekov leaves the Enterprise, and Arex and M'Ress join the crew. But, honestly, for the story itself it doesn't really matter.
A couple of years ago, the Enterprise helped a Syhaari ship that was stranded in deep space - one of the first ships of that species to venture into space travel. Now, the Enterprise returns to their homeplanet to formally start negotiations. They are quite surprised to discover that the Syhaari space engines are now far more developed, and designed quite differently than just 2 years ago... so what happened? And to make matters more difficult a giant planetoid appears to enter the star system, and not peacefully.
The main point of criticism is that this book offers little in terms of surprise, the plot was rather predictable from start to finish which is the reason why I'm rating this only with 3 stars - I just had the feeling I've read a plot like this before... multiple times. Nonethess, Swallow manages to tell a gripping tale about ambition, racism, caring only for self-advancement, mindless revenge - and of the overlying ability to collaborate and compromise to the benefit of all. It's easy to see parallels to present times. The characterization of the main characters is top-notch, but that's what I expect of a writer like Swallow. So if you're willing to overlook the predictability-issue and like the TOS-setting and crew, you're in for an interesting story.