Not far from the altar, along the outer flank of the left aisle of the cathedral, is a room in which architecture, sculpture and painting meld in an absolutely unified whole. This space is known as the Piccolomini Library. Here, according to the intentions of Cardinal Francesco Piccolomini, was to be housed the invaluable family collection of books and manuscripts, assembled mainly by his uncle Pius II, the great humanist and himself the author of literary works. In reality the Library never held these volumes but became a magnificent reception room; therefore it was never used for services of worship but wholly devoted to the glorification of the figure of Pius II.

Work began in 1492, when some rooms in the old sacristy were united into one large space covered by a pavilion groin vault; the lower part of the walls was lined with wooden benches carved between 1495 and 1496 by Antonio Barili (later modified and partially destroyed in the eighteenth century). In the center of the room was placed the beautiful marble sculpture group of the Three Graces, an ancient Roman copy of a Greek original, bearing witness to the humanist interests of the Piccolomini family. Then in 1497 the Sienese artist Lorenzo di Mariano, known as Marrina, created the elaborate marble entrance façade separating the library from the left aisle of the church. But it is the painted decoration, commissioned . . .