I also really loved this series, and I'm excited to get my hands on Water Devil. I've read Vision of Light several times, and Green Lion only once, so I think I'll have to read it again before I move forward. I also really like the mood, setting and characters, and think that this book compares favorably with The Pillairs of The Earth by Ken Follett in that regard. While Pillars of the Earth has more detail about politics and architecture, it also has a lot more detail about sex and rape, which makes Vision of Light look very tame. To deal with that, I had to make my own personal expurgated version complete with torn out pages and scribbled out words and scenes, because I like to read the story.
I also wanted to mention that for all their arguing, Margaret and Gregory have remarkably similar beliefs about the nature of God. When Margaret had her Vision of Light, she essentially did successfully what Gregory was trying to do through a life of contemplation. She was humble, she was physically weak, she was meditating on the nature of God, and was emptied of all that distracting sense of self. Her "mind was blank and empty as an open grave." In this state, she saw the light, and was overwhelmed by a feeling of ecstasy and felt like she was one with the universe. This is textbook mysticism (see
http://en.wikipedia.org/wiki/Mysticism especially the section titled Process). Different characters in the book have different relationships with the God, but it was pretty clear to me what the authors views were on the subject. I was always a little annoyed with the way that Riley's consistant image of God didn't quite mesh with my own. (Now that you point it out, though, I can possibly believe that she was doing it as an artistic device to juxtapose Margaret's success with Gregory's failure within the tradition of the time, and that it may not be her own personal view. I'll have to think about it.)
Anyway, I'm babbling. I think people who like Historical Fiction, and don't mind a little romance thrown in would like this book. It has a lot to offer.