The Guarnacci Library originated in the 18th century with the legacy of around 7,000 volumes belonging to Mons. Mario Guarnacci, to which other donations were added over the years. Located in Palazzo Vigilanti, it currently possesses some 50,000 volumes, including manuscripts, codexes, illuminated manuscripts, incunabula and sixteenth-century editions. Noteworthy among the repertories are the Diplomatic Archives (10th-19th centuries), the Maffei Archives (14th-19th centuries), the Archives of the National Guard (1860-70), the Pacciani collection (19th century) and the Funaioli collection (19th-20th centuries).

The Library of the Accademia dei Sepolti is also kept here on loan. It includes the Amidei Herbarium, compiled by Gaspero Amidei in the mid-19th century. The herbarium is made up of specimens of the flora found in the Val Tiberina, in the Lucca Apennines and in the areas around Pisa and Volterra. It contains some samples coming from the older Herbarium Chiostri, compiled by Lorenzo Chiostri. The herbarium was donated by Amidei to the Volterra Agrarian Assembly, from which it was later acquired by the Accademia dei Sepolti. Originally consisting of over 2,000 samples, it was reduced to 850 already by the end of the 19th century, due to its poor state of conservation, but it is still today considered the most important collection of plants from the Volterra area. During the two-year period 1984-86, the herbarium underwent restoration.