It did a good job showing the events of Mary I's reign from the angle of how Elizabeth and Mary were interacting, but the book skips over a lot of Mary and Elizabeth's more formative years.
The book starts with Elizabeth being born, so we don't get any of Mary's up and down childhood that contributed to a lot of her actions later, and Anna of Cleaves and Katherine Howard are, combined, covered in just one paragraph, without showing what impact those two very different "step-mothers" each had on Elizabeth and on Mary.
So, a good book on Mary I's reign through a new lens, but not as complete as it could be.