The Other Boleyn Girl by Philippa Gregory
The other girl is Mary Boleyn, the sister of Anne of the Thousand Days. Mary was Henry VIII's mistress and gave him two children before the Boleyn and Howard family's ambition drove them to replace Mary with Anne with the goal of making her queen. The plan was a success with disasterous results.
At age 14 Mary is already married but catches the eye of the king. After an affair lasting several years, Henry's eye wanders to Anne. But Anne holds out until Henry decides to divorce Queen Katherine and make her queen. After splitting from the Catholic Church and ridding himself of Queen Katherine the two are married. Anne's life becomes a desperate quest to conceive a son. After the birth of the future Queen Elizabeth I, Anne has a series of miscarriages and Henry begins to wander toward greener pastures.The book dwelves into politics, romance, duty and family. Gregory brings the era to life in a way I've never pictured it. A king's court had always seemed such a romantic life to me but, after this book, I don't think so anymore. Court was treacherous. Everyone was trying to further their own agendas in a quest for royal favors. People were pawns. Life was a struggle to remain in favor. No one in the book came off as trustworthy or sympathetic. It wouldn't be a life I would have wanted.