Born to Isabel and Ferdinand, the Catholic Monarchs whose marriage united the realms of Castile and Aragon, Juana -the Mad- (1479-1555) is one of the most infamous but least studied monarchs of the Renaissance. Conventional accounts of Juana portray her as a sullen woman prone to depression, a jealous wife insanely in love with her husband, and an incompetent queen who was deemed by her father, husband, and son, unable to govern herself much less her kingdoms.

But was Juana truly mad or the victim of manipulative family members who desired to rule in her stead? Drawing upon recent...