Katherine of Aragon - series of books about the wives of Henry VIII, and I have to say I ADORE the covers. Henry gets progressively older in each one, which is fantastic since most show him as old fat Henry all the time.
The Memoirs of Mary Queen of Scots by Carolly Erickson. She is cousin to Elizabeth I and we get to see their life long rival to each other. Also a bit of a love story and her courage through a difficult reign.