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The Gilmore Music Library has thousands of rare published works, including both books and scores. They are cataloged in Orbis.
Here are a few highlights. The links will take you to the descriptions in Orbis.
Theorica musice, by Franchinus Gaffurius, a theory treatise published in 1492.
A partbook for a set of masses by Pierre de la Rue, published in 1503 by Ottaviano Petrucci, the famous pioneer of music printing.
Gioseffo Zarlino, Istitutioni harmoniche, the most influential theory treatise of the 16th century. The library has two copies of the second edition (1573), one of which is signed on the title page by Claudio Monteverdi.