A work is in the public domain if:
Once a work falls into the public domain, it is no longer copyrightable and anyone may use public domain works without the permission of the copyright holder, who is no longer entitled to the exclusive rights of a copyright holder. These works can be excellent choices for use in teaching and research because there are no permissions required for their use.
Ascertaining whether or not a specific work has fallen into the public domain can be difficult. The only general rules of thumb are that works published prior to 1923 and unpublished works created before 1890 are have fallen into the public domain. The Digital Copyright Slider can help determine when something is or may be under copyright, and when it is in the public domain.
Works for which you cannot ascertain or contact a copyright holder are called “orphan works” and are not currently available for use. The Google Books settlement is attempting to create a registry, which should allow the public to access orphaned works through the Google Books search platform. Sources for public domain material and research on the public domain:
NOTE: It is important to be aware of the distinction between materials in the public domain and materials that are open access. Items in the public domain are free from copyright restrictions whereas open access content is available online without access restrictions. Open access material may still be (and probably is) under copyright.