Special Collections and Rare Books houses rare and unique materials that span over four thousand years. The collections include books, maps, posters, pamphlets, comics, artwork, artifacts, personal papers, and over 8 million titles on microform. Many of these materials are valuable primary sources.
The materials in Special Collections are here because they are fragile, one-of-a-kind, and they have lasting value for research. Because of this, the procedures for using books and other materials are a little different from what you may be used to in libraries. This guide will walk you through the process of finding and requesting materials in Special Collections.
From "What is a Primary Source?" http://www.princeton.edu/~refdesk/primary2.html.