Priestess of Avalon by Marion Zimmer Bradley and Diana L. Paxson is the story of a priestess named Helena who was forced to leave her home at Avalon and live an entirely different life from what she was used to. The story follows her life and her pseudo marriage and tells of the different places that she lived in the Roman Empire. The story tells about her relationship with her children and her grandchildren and also about her feelings in regard to the rise of Christianity in the Empire and how it relates to her own spirituality. There are some parts of the book that were enjoyable for me such as Helena's time on Avalon and reading about how her spirituality evolves in her time away from Avalon, but I found much of the story to be very tedious to get through and I'd have to say that this is probably my least favorite book in the Avalon series. I'd probably only recommend reading this book if you are also reading Lady of Avalon because the story of Priestess of Avalon overlaps, both chronologically and in plot, with part two of Lady of Avalon, though I'm not sure it's entirely necessary.
There are many options for the order in which to read the two books. You could simply read Lady of Avalon first and then Priestess of Avalon second or vice-versa. You could read Priestess of Avalon anytime after the first part of Lady of Avalon, but make sure you finish reading it before you start part three of Lady of Avalon. My preferred method is a bit more complicated though. First read part one of Lady of Avalon. Second read part one of Priestess of Avalon and up to chapter 11 of part two. Third read part two of Lady of Avalon. Forth finish parts two and three of Priestess of Avalon, and then finally read part three of Lady of Avalon.
Priestess of Avalon(along with Lady of Avalon) takes place after The Forest House, so you might want to read that book first as well as Ravens of Avalon which provides some of the background story to The Forest House. There are also some references made to The Fall of Atlantis and Ancestors of Avalon, but I'd say it's less important to have read those books prior to reading Priestess of Avalon. One last thing I'd like to comment on is that this book is written in first person while the other books in the series are all written in third person(with the exception of some introduction passages). I'm not a big fan of first person perspective, but I have to say that it was done well enough in this book that I didn't really notice too much.