It is a relatively decent "Star Trek" book, and the tie-in to an original series episode is good world-building, as is the decision to weight the story in favor of Uhura, Sulu, and Chekov, as opposed to the "usual" mainline stars of the franchise. Unfortunately, the characterizations of the series characters just feel very slightly "off-model", in a manner of speaking. They are certainly recognizable, but it just feels every so slightly ... well, "off-model". It is not enough to make the book unreadable, but it is just enough to be a nagging sort of itch that you can never really scratch.
The story line is also a bit muddled - it sometimes seems as though there are a few too many hands in the kitchen, as the book seems as though it is simultaneously mashing together two stories - one focusing on the Elasians, and one focusing on the potentially staggering scientific possibilities of volcanic lifeforms - and neither one totally seems to get quite the weight that it deserves, although of the two, the Elasian plot carries most of the weight, and gets the best resolution.
As "Star Trek" books go, this is not at all bad - the setting and time period are pretty good, the choice to tilt the narrative focus away from the "usual" mainline stars of the series works well, and the tie-back to an existing episode (and a deeper dive into a one-off alien culture) are definite high marks in its favor. It just hovers a little bit more on the side of being a good "Star Trek" book, as opposed to a great "Star Trek" book - but it is definitely worth the time to read, at least once.