Just finished up "The Captain's Oath" and I rated it above average. It's not your typical book with a single narrative (or the occasional book that might have 2 overlapping plot threads). This is almost what I'd call an anthology, but with the common thread of giving us some insight into James T. Kirk as a captain. Sometimes there's a danger when an author tries to carry on too many storylines that some things may be lost in the story they hope to tell. I did not notice any issues with that here. Christopher handled the various storylines pretty well, giving each adequate attention and bringing each to a satisfying conclusion. My main criticism isn't related to the stories, but to the jumping back and forth through time. Sometimes that led to a bit of confusion as I had to recall things that were going on at a particular time frame when we'd return to it.
My favorite storylines had to do with the Agni, a species that resides in an N-Class environment (also known as Demon class--as noted in Voyager) that the Sacagawea encountered, and the Aulacri storyline that the Enterprise encountered.
It is clear in the book that Christopher does not view Captain Kirk as a maverick, cowboy style captain in his early days. He drives that point home in the novel a couple of times. This is not the Admiral Kirk we saw in Star Trek III: The Search for Spock. This is a captain who views rules and regulations as very important. Yet he does have a gift of knowing how to interpret the rules. He doesn't break the rules, but he may bend the letter of the rules if he feels he needs to respect the spirit of the rules. That is what makes Kirk not just a good, competent captain, but a great captain, and one day legendary. A gift of looking at the meaning and intent of the rules and regulations. As time goes on he becomes even more adept at that skill.
I noted earlier in the thread that I was initially doing a compare/contrast with "Enterprise: The First Adventure", since that book also features Kirk taking command of the Enterprise. However both are very different novels. I see very little in common between the two, other that Kirk had wanted Mitchell to be his first officer and Pike had used his leverage to have Spock become first officer (which has been noted in other stories as well) and a few very minor points that aren't even worth mentioning. The only other possible similarity was in E: TFA Scotty believes Kirk to be basically an untested captain and not worthy of the Enterprise. In TCO it's Kelso who sort of takes that role on, though not as overtly. He is not sure Kirk is worthy of the Enterprise. It's interesting to note that Christopher has Kelso as a holdover from Pike's command (though for just a few months) similar to Piper being a holdover as well. I always wondered about Piper (even creating a poll about it at one point), but I have to admit I never gave much thought to whether Kelso and others on WNMHGB might have been holdovers as well.
I did find some common elements with the earlier "My Brother's Keeper" trilogy which I read about a year or so ago. Not as far as plot points, but in how Michael Jan Friedman and Christopher viewed Kirk's early history. That Kirk was not the maverick in his early days that he is sometimes made out to be. Kirk is very committed to the rules and regulations and Mitchell is the one trying to get him to relax more in both works (though MBK, by it's focus, gives more equal time to Kirk and Mitchell). I would say the first two MBK novels could conceivably fit to a general extent with Christopher's story (not in every detail, but Friedman gives some information in Kirk's pre-command career which I enjoyed and Christopher gives information about Kirk's early command which was equally enjoyable). The 3rd MBK novel however does not fit all that well, storywise or continuity wise.
But overall I enjoyed the book. It was nice to see stories of Kirks first command. I admit I sort of just thought the Enterprise was his first command for a long time. I didn't take issue with it when I heard Christopher was writing about it...I just never gave it a lot of thought. But even E:TFA had Kirk in command of a prior vessel before the Enterprise (though at the rank of Commander officially). Christopher does have the common refrain that Kirk is the youngest person to make Captain in Starfleet up to that point, it just was of another ship. And maybe a bit of a nod for doubters does have Kirk being the youngest person to be given command of a Constitution-class starship at that point. It was also nice to see a new story of Kirk taking command of the Enterprise. For years novels have flirted with him taking command but never really touched it, leaving E:TFA as a story of the first mission of the Enterprise....until now. It's nice to see a novel more consistent with WNMHGB.