Every major library has a special collections section or department. The Harold B. Lee Library at Brigham Young University has an extensive archive with a huge underground preservation vault. The L. Tom Perry Special Collections Library has some surprising collections.
Special collections libraries often collect the "papers" of historically important or interesting people. Usually, in a university or college library, the decisions to add documents to the library's collections is made by the curators, usually professors, at the school. You cannot assume that your family was "too poor" or "too obscure" to have important genealogical documents preserved in a university special collections library. For example, my Great-grandmother accumulated a huge collection of genealogically important documents during her lifetime. Normally, those documents would have been lost to her posterity. But, I made the effort to convince the BYU Special Collections Library to take all the documents. There are over 16,000 names of people she collected in those documents. Here is the catalog entry for the Mary Ann Linton Morgan Papers.
Here is the description of the documents from the Library Catalog:
The Mary Ann Linton Morgan family papers contains geneaological information and pedigree charts compiled by Mary Ann Linton Morgan. Also included are letters from 1869, 1878. Old family trees of the Sutton family are included. A diary from 1924 is contained as well as the patriarchal blessing of Mary Ann Linton Morgan. In addition, there are two letters to the family of John Hamilton Morgan from Heber J. Grant. Missionary photographs from the 1930s in Tonga are included from an Elder Vincent. The collection contains documents from 1869-1990 but primarily consists of materials from circa 1930-1950.These "papers" are genealogically and historically valuable. A more complete description of the papers is contained in the Manuscript Collection Descriptions.
The idea here is simple. Special collections libraries might have some valuable documents relating to your family history that are "mixed in" with a collection from someone who lived at the same time and in the same place as your family members. You will never know what is there unless you look.
Another way to approach the Special Collections library is to use ArchiveGrid.org. Here is a description of the ArchiveGrid from the website:
ArchiveGrid includes over four million records describing archival materials, bringing together information about historical documents, personal papers, family histories, and more. With over 1,000 different archival institutions represented, ArchiveGrid helps researchers looking for primary source materials held in archives, libraries, museums and historical societies.
Here is the entry for the Mary Ann Linton Morgan family papers from ArchiveGrid.org.
Here is another entry for another of my ancestors.
Of course, I am using my own ancestors who lived in the Utah/Arizona area. But there are special collections libraries in every part of the United States and many foreign countries. These libraries have collections of documents that may include many of your ancestors. I could keep going with examples of people with huge collections. But here is one more example using my surname and searching in ArchiveGrid.org.
For the first parts of this series see: