Saint Louis University offers exceptionally rich library holdings. The Pius XII Memorial Library holds approximately 2.5 million volumes. It contains all of the major source collections, research tools, and scholarly journals essential for graduate study in Medieval and Early Modern European history. These include the electronic version of the Patrologia Latina as well as thousands of rare books dating back to the fifteenth century. Pius XII Library also contains on microfilm virtually the entire Vatican Library, a resource found nowhere outside Rome. The Vatican Library holdings consist of approximately 37,000 medieval and early modern codices. The university library is also a federal government depository.
Students also have access to the Missouri Historical Society, which holds 70,000 volumes and manuscript collections relevant to the study of the American West, and the Mercantile Library, a private subscription library, the oldest west of the Mississippi, which contains over 300,000 volumes, 350,000 photographs, and manuscript collections relevant to westward expansion, Native American history, and rail and river transportation. Additional library collections are found at the Missouri Botanical Gardens, the Center for Reformation Research, and other university libraries in the metropolitan area.