I want to give it a two for how I much I enjoyed the story, and three for how much I enjoyed the character... I'll be a pessimist and go with the lower grade. 
 I remembered only the vaguest details of the original books, which did sort of leave me at a slight disadvantage, but only of the over-arching historical details. The narrative didn't overlap except for the tiniest bit with the previous book. For the historical details, I went off to Wikipedia and Anne of Brittany...which, I have to say, ended up kind of depressing in terms of where the story is going to go with the Duchess for the next book. From the perspective of the protagonists, things don't really end in a happy way for them if we follow the events as they happened. 
 I think I enjoyed Sybella a lot better than Ismae as a character, her broodiness appealed to me. But for the first half of the book I also found the story very boring...and plodding. Things do perk up around the middle of the book though.