Though Moon visits an old world of hers in this book, it feels very much like a new series. I enjoyed her Paksennarion books years ago; I think I enjoyed this one even more.
Oath of Fealty has a slow start, and doesn't really pick up until thirty or forty pages in. But in a close to five-hundred-page-long fantasy epic, that's forgivable, and there's really very little else to dislike about this book. Once it gets going, it really goes, following three main characters: a newly-crowned king who only recently learned about his royal heritage, a mercenary who has suddenly inherited that king's old military command, and, most compellingly, a noblewoman who has been commanded to go and restore order to her treasonous family's duchy.
This is definitely swords-and-sorcery stuff, but unlike some fantasy, the evil beings aren't nearly as compelling as the strong and good gods or the imperfect humans who sacrificially serve them. This is good fantasy, and while it doesn't have quite the theological bent of Bujold's Chalion books, it's in that mold.
At one point, the noblewoman, Dorrin, when speaking of her first few weeks trying to undo the harm her wicked family has done to their land and people, complains, "[The people's] gratitude is too great for the little I have done so far. It is all undoing - undoing curses, unsetting traps - before I can do anything real". Though it's pointed out to her in the same conversation that she has been able to do positive good, I have to say that that's what I really hope to see out of the next few books of the series: I want to see what these characters build in their new realms of responsibility. I'm sure those volumes will involve more fights with evil dukes, evil bandits, evil armies - that's where the fun of the story is, after all - but Moon's an author that seems to have a good grip on, well, good, and I'm hoping that in the later volumes of this series we'll get to see Dorrin and the rest not just destroy the enemy, but build up their new homelands. It should be interesting.
One note: though Moon isn't at all a writer that terrifies for the sake of terrifying (this isn't horror), her bad guys are really bad, and so this isn't for young readers, because it does contain some disturbing elements of violence and, well, creepiness. I think that it makes good reading precisely because the bad guys are really bad and the good guys really good, but fair warning that some of it is hard going.
Peace of Christ to you,