Andover-Harvard holds about 30,000 volumes of rare books, tracts and other rare materials. Most were printed before 1851. The collection includes 23 incunabula or books printed during the earliest period of printing with movable type, roughly from the time of the printing of the Gutenberg Bible to the early 16th-century. The oldest is a book on virtues by Guillelmus Paraldus printed "not after 1475." The Andover-Harvard collection is often complementary and supplementary to the half million volume collection at the Houghton Library, Harvard's main rare book repository, and rare book researchers will often need to consult both collections.
Andover-Harvard's holdings include early Hebrew, Latin and Greek bibles as well as many bibles in vernacular languages printed on missionary presses. There are first editions of Luther, Calvin, Melanchthon, Zwingli, and other reformers, including a large collection of 16th- and 17th-century Dutch and Latin tracts by Jacobus Arminius and his followers who were early opponents of strict Calvinist theology. There is an excellent collection of materials concerning the 18th-century Salzburg Protestants, a Lutheran refugee group expelled from Salzburg, Austria in 1731 and 1732. One will also find early editions of the works of the New England Puritans and first editions of most of the works of important 19th-century Unitarians and Universalists, such as Channing, Parker, Ballou, and Emerson. Included also are the libraries of such notables as Bishop John Codman of Dorchester (1,250 vols.).
Additions to the collection are made occasionally by purchases from designated funds or by the receipt of gifts.