It's a cool story, and told in a pretty unique way--It starts where Where No Man Has Gone Before ends, and Captain Kirk is thinking about the death of his friend, Gary Mitchell.
I once tried to write a story where I had to watch Where No Man Has Gone Before once through, and then again, stopping every few seconds. It made me realize three things: one, that I definitely wasn't doing this for every episode as I had originally planned; two, that the story was really, really creepy (something I'd missed--though I have no idea how--the first time I watched it); and three, that Gary Mitchell was a very infuriating character to write--he has this way of acting, but you can never tell what he's really thinking.
Also, most of the time in the episode he's playing an evil god-like being with weird silver eyes, so I kept wondering how much I could even trust the impressions I was getting from the episode--was he always, secretly, evil; or was he an ok guy before he had the misfortune of being zapped on the way through the Energy-Barrier-at-the-edge-of-the-galaxy?
The trilogy is mainly about two people: James T. Kirk, and Gary Mitchell. But most of all it's about Gary Mitchell. Who he was, and the influence he had on Kirk. As it is designed being the memories of Kirk, of 'turning points', you might say, it lets each book be about what he learned without ending up feeling like it was shoved in there, or that everything happens annoyingly 'just right'.
The other cool thing about the books, is that you are never bored for more than one or two pages at a time, and rarely that; and both stories--the frame story and the memories--hold your interest equally.
Each book is it's own, separate adventure, so to speak, but there is a thread that holds it together--a strange mystery that only gets revealed in the third book. After so much building up to it, I thought that anything it would be wouldn't hold up to my expectation, but to my surprise, it did--and it was something I'd never thought of.
That might not be saying much, since I can never solve mystery stories before they're explained at the end, but it did seem unexpected and at the same time, it made sense.
So, really, it did a lot of things well--the plot(s) were interesting; the characters seemed like themselves, but seen from an angle you never saw in TOS; and the writing was good--sometimes even beautiful. All in all, it's worth reading, if you ever wanted to know more about Gary Mitchell, or why, on his friend's gravestone, he wrote 'James R Kirk' instead of T.
(though, to be honest, don't look forward to that too much--I did, and then I was disappointed. I thought it would have some 'profound and clever meaning' to it. It doesn't.)
Book One: Republic. Book Two: Constitution. Book Three: Enterprise.