One of the most famous examples of her books is The Other Boleyn Girl, about Anne Boleyn and Henry VIII, told from the point of view of Anne's sister, Mary, who was Henry's mistress long before Anne was. But I actually enjoyed The White Queen much more, as it brought to life a lesser known historical figure and period. Thanks to popular culture and TV shows such as The Tudors, everyone knows about Henry and Anne, but fewer people know who Elizabeth Woodville -- Henry's grandmother -- was. The way Gregory writes it, though, Elizabeth Woodville was a strong woman and an influential figure in England's history. It's too bad her grandson is better known for beheading his wives than she is for her contributions!
I'm sure that there are many liberties taken with this kind of novel -- the author has to fill in the gaps, bring the characters to life by supposing what their thoughts and conversations might have been about. But I'm sure you'd also find, if you checked, that certain authors -- like Gregory -- do quite a bit of research to make sure their novels are as close to fact as possible.
Can you think of other authors who write historical novels that have an almost biographical quality to them? Who are your favorites?