Somewhere, in the boxes upstairs, I have the books of the Deed of Paksennarion, which directly precedes the story related in 'Oath of Fealty' and 'Kings of the North'. (I'm going to talk about 'Oath' and 'Kings' together, since they're not particularly separate entities.)
Moon's introduction specifies that she considers these to be independent of the previous trilogy, and that a reader can start here.
However, I did wish I'd read the earlier books first. They apparently contain many of the same characters, and I really felt I'd have gotten into the story and felt connected with the characters a lot faster if I'd already known their background.
That said, these are very well-done books. They take place in a familiar European-based fantasy world, but the scenarios and characters are realistic, believable and well-drawn. Moon is well-known as a writer of military fiction from a woman's perspective, and that's what we get here. It's not all women, but my favorite character was probably Dorrin - a somewhat-older, capable veteran who happens to be a woman.
The hierarchical social system accepted in this world isn't one I'd particularly want to live in, but the story isn't about ideal worlds; it's about people trying to do their best in the world they've got.
My one issue with it was probably the absolutes of good and evil - one of the major plot points is that an entire family is Pure Evil and must be eradicated. Probably to balance this, there was a sub-plot about one culture thinking that another is evil due to cultural misunderstandings, but I would have liked to at least have seen the perspective of someone within the family of Evil Sorcerers.
It's also not a particularly tightly plotted or suspenseful story - it's more about enjoying the twists and turns of the complex politics and personal maneuvering of the characters. Like real life, more things keep happening... and I liked following them.