The Deadly Sisterhood A Story of Women, Power, and Intrigue in the Italian Renaissance, 1427-1527 by Leonie Frieda (nonfiction)

I've read Frieda's biography of Catherine de Medici and this is just as good, telling the story of various women from the generation before her.  These women were often left as regent of their husbands' or sons' city states because of the turmoil of the times.  Turns out the Italian Renaissance was also a renaissance for women, even if it didn't last long.