Scholarship on Italian women’s secular writing of the sixteenth century has illuminated the remarkable success female authors enjoyed in print, as well as the complex and ambivalent responses they evoked as a group. This article argues that the more shadowy praxis of ordinary female letter-writing, an obligation for most elite wives and widows, required a baseline level of literacy that enabled more eminent literary women to flourish in print. The essay studies the unpublished letters of the Roman noblewoman Costanza Colonna, the Marchesa of Caravaggio (ca. 1556–1626), to demonstrate her development as a self-taught, competent writer, familiar with the terms of Renaissance debates on the epistolary genre. Colonna exemplifies the hidden world of female literacy that helps explain both the extraordinary flourishing of women’s publishing in sixteenth-century Italy, and the hostility it encountered from some elements of society.