Academic libraries and other research institutions often maintain "special collections," "rare books collections," or archives that collect rare or specialized materials on a certain subject. Such collections are important to researchers and scholars.
Finding rare-books and archival collections related to your topic:
Mindmap: Useful Keywords for Searching Library Catalogs (see "primary sources")
Finding Primary Source Documents (UNC-Chapel Hill's tutorial)
Indexes to special collections:
Rare-books collections and archives typically collect materials in a general subject area; within that, they often focus on a geographical region or time period. (For instance, ZSR's Rare Books Collection specializes in "American, Irish, and British authors of the eighteenth through twenty-first centuries"; our Baptist Historical Collection collects materials relating to Baptists in North Carolina.) So if you're taking a subject-based approach to identifying collections, you can use a moderately broad search strategy, adding keywords to reflect any geographical, chronological, or other aspects of your topic.
There are also collections devoted to a single person. So if your topic centers around a specific writer, artist, historical figure, etc., try searching their name combined with keywords for collections (see Lecture Points).