Visit their home pages. Have they digitized materials? If they haven't digitized, have they published Finding Aids (which give you a good sense of what materials are contained in their collections)?
When searching a database of books, whether the catalog or Google Books or WorldCat:
Oral histories (usually collected through interviews) are an increasingly popular method to preserve the past. Examples are below:
The U.S. government is one of the most prolific publishers in the world. Nearly every aspect of life is touched by government activity, and it is a publisher of all types of materials (statistics; reports; legislation; etc).