Works in the public domain are those that are never protected by copyright (like facts or discoveries) or works whose term of protection has ended either because it expired or the owner did not satisfy a previously required formality. Currently, all pre-1926 U.S. works are in the public domain because copyright protection has expired for those works.
From Cornell University Library. This is a chart that shows when an item is no longer under copyright and is considered to be in the public domain.
Public Domain Resources
This DACC Library research guide provides access to articles, journals, eBooks, eTextbooks, images, audio recordings, videos, and more that are in the public domain:
Doña Ana Community College