The Tigress of Forli, Renaissance Italy's Most Courageous and Notorious Countess, Caterina Riario Sforza De' Medici by Elizabeth Lev (nonfiction)

Learning about this woman was amazing - she wore one of the only cuirasses made for a woman at this time.  It had reinforced plates so her breasts would not be crushed in battle.  The author uses plenty of primary sources, some from the Countess' letters that have never been translated into English before.  She fought against Cesare Borgia - need I say more?