The Lady Queen: The Notorious Reign of Joanna I, Queen of Naples, Jerusalem, and Sicily by Nancy Goldstone (nonfiction 2009)

Joanna was a unique woman in power at the time, and went through quite a bit to hold on.  This book helps shed light on the political structures of the time.  Reading about how Joanna was successful and not-so at navigating the medieval world's touchy relationship with royalty and women is wonderful.