The only Shatnerverse book I ever bought was Ashes of Eden. At the time, I really enjoyed it. I've listened to the audiobook versions of book 2 to book 9 (Collision Course doesn't have one), and I agree that Shat's books are very Mary-Sue. With that being said, I still enjoyed the stories no matter how over the top they were. The best thing about them is the fact that Shatner reads them himself. It's like Kirk-prime lived on and told us the stories himself
I like that
. I was very unhappy (like a lot of people) and not satisfied with the way that Ron Moore ended with Kirk in Generations. It didn't feel like it was the right way to end with that character (not to start up that old debate again). I think the whole idea behind this series, for me at least, is a little fantasy that one of the greatest captain's in Starfleet history got a little better of an end story than the nonsense ending of Generations. Even if it doesn't fit in with the rest of Trek-lit, I still enjoy it on that level.
Then you might like David George III's Kirk volume in his Crucible series. It takes on those events in a novel fashion, but without making Kirk superhuman, while still giving his story a great ending.