The Library of the Convento dell'Osservanza, whose original collection was formed in the first half of the 15th century, is one of the city's richest. It is composed of about twenty-five thousand volumes, among them hundreds of precious sixteenth-century editions. It boasts a priceless group of incunabula made up of texts on philosophy, theology, canon and patristic law, in addition to numerous Latin and Greek classics. Noteworthy from the historical-scientific viewpoint is one of the first editions of the Encyclopédie of Diderot and d'Alembert, as well as a large number of treatises on botany, zoology, physics, chemistry, geography and cosmography dating from the 17th to the 20th century. The catalogue of the Osservanza Library has now been entered in the computerised system of the Sienese Library Service.

The Museum of the Osservanza, established in 1920 and dedicated to Father Aurelio Castelli, occupies premises annexed to the convent. Among the valuable objects displayed in the museum (paintings, sculptures, seals, illuminated manuscripts) is the precious treatise De animalibus by Albertus Magnus (13th century), the first page of which was illuminated in the second half of the 15th century by the young Francesco di Giorgio Martini.