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.
In the center of the library hall is a marble antique group, the Three Graces, a 3rd century Roman reproduction of an ancient Greek original. The sculpture was purchased by Francesco Todeschini-Piccolomini in Rome and was probably used by Raphael for his painting on the same theme.