According to the U.S. Copyright Office, "A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner."
Under current U.S. copyright law, every January 1 until 2073, works published before 1978 enter the public domain:
After 2073, copyright will expire 70 years after the death of the creator.
Many of these public domain works are available through Hathi Trust, Google Books, and Internet Archive.