The Forest House by Marion Zimmer Bradley is one of the books in her Avalon series. The story is set during the Roman occupation of Britain about a generation after the uprising of Boudicca. Much of the story is centered around the Forest House, which is the sanctuary for the Druid priestesses that was established after the destruction of the Druid complex on the isle of Mona by the Romans, and the woman who serve there. All of the books in this series have elements of romance, but it is a particularly strong theme in this book and a major plot point is the forbidden love between Eilan, a Druid priestess of the Forest House, and Gaius, a half-Roman soldier. The story follows their lives parallel to each other and the few times they are able to meet. Although they can never truly be together, in their own ways they both work to try to keep the peace between their two peoples as certain British factions vie for revenge against the Romans for the atrocities they committed years before.
This book takes place after Ravens of Avalon and I would strongly suggest reading that book before reading The Forest House. Ravens of Avalon provides much of the background story for The Forest House and I definitely felt less confused about events mentioned in The Forest House on this read than on previous reads before having read Ravens of Avalon(despite some minor inconsistencies between the two books). You might also want to read The Fall of Atlantis before reading this book in order to understand certain references, but it's not wholly necessary. I wouldn't say that The Forest House is my favorite book in the series, but I did enjoy it and would recommend it to other fans of the series and to people who enjoy British history.