María de Castilla (1401–1458), daughter of Enrique III de Castilla (1379–1406) and Catalina de Lancáster (1373–1418), was a highly influential political leader and patron of culture. Queen of Aragón through her marriage to Alfonso V (1396–1458), she governed as lieutenant general in his absence, wielding greater powers than any of the previous Aragonese queens and full sovereign authority equivalent to Alfonso. María’s diplomatic acumen was demonstrated during key moments such as her intervention on the battlefield of Cogolludo (1429), earning her a reputation as a peacekeeper. She was a passionate patron of the Franciscan order, founding monasteries and supporting female religious figures like Isabel de Villena, the author of her own Vita Christi. María’s extensive correspondence reveals her engagement in both political and personal matters, showcasing her advocacy for women’s work and welfare. Her library, rich in devotional and mystical texts, serves as a further testament to her value of devotional literature and encouragement of religious literacy among the women in her household.