The Other Boleyn Girl by Philippa Gregory. If you're a fan of the new Showtime series
The Tudors, you might like this book. It tells the story of Henry the Eighth's second and turbulent marriage to Anne Boleyn, but through the eyes of her younger sister, Mary. Mary was a real person, and she was present at the court of Henry, although not much is known about her. Gregory has used a combination of historical facts and her own imagination to create a detailed picture of 16th century England and its volatile royal family. This book was also recently made into a film of the same name by director Justin Chadwick with Natalie Portman and Scarlet Johansson as the two sisters.