I haven't read that particular book -- actually I haven't been able to read any of her books, and she's become quite prolific. When "The Other Boleyn Girl" first came out I was struck with a basic historic inaccuracy in everything I read about it, mainly that Anne and her sister Mary were in direct competition for Henry VIII's affections.
No. They weren't. Period. To portray it that way is so wrong that I couldn't bring myself to read it.
I did see Gregory when she came to Kansas City and spoke at a Rainy Day Books event (wonderful bookstore) about her then-latest, "The Constant Princess" about Katherine of Aragon. She had tried very hard to get into Katherine's head, but from the part she read to us I was clearly hearing a substantially modern person. Gregory isn't quite as bad as some, because she does do her research, but I think she tries too hard to make her characters somehow relevant to modern readers, as if the only differences between us and them really is the clothes we wear and perhaps the food we eat. No. It's far more profound than that. Which is why I keep coming back to straight biography, because usually those authors are trying very hard to understand their subjects, and get it that there are more differences between us than we can fully know.