Marion Zimmer Bradley's Ravens of Avalon by Diana L. Paxson is about the Roman conquest of Britannia and their oppression of the Druids and the Celtic/British people in general. The story follows the lives of the Druid priestess Lhiannon and the Iceni princess Boudica and how they worked to fight back against the Romans. If you are familiar with the history behind this story, then you probably already know how it ends, but even so I think the story is worth reading because it's more about the journey than the destination. This book is slow paced like others in the series but I can enjoy a slow pace if the writing is good which it is for the most part. There were some parts in the middle that I found myself wondering what the point of the chapter was as it didn't seem to add anything to the story, but things did pick up again and got more exciting. I would definitely recommend this book to fans of the Avalon series or fans of Celtic/British history.
This book takes place after The Sword of Avalon, but it's not necessary to read that book(or any of the books that take place before it) prior to reading Ravens of Avalon. Ravens of Avalon does take place before The Forest House though, and I would recommend reading it before reading The Forest House as it provides a lot background story to what is going on in The Forest House.