In 1492, the Archbishop of Siena, Cardinal Francesco Nanni Todeschini-Piccolomini (later Pope Pius III), founded the Piccolomini Library (Libreria Piccolomini) in the cathedral to store the most valuable family books and manuscripts with colorful miniatures from the collection of his uncle Enea Silvio Bartolomeo Piccolomini (Pope Pius II). The library room adjoins the left nave of the cathedral.