Please note: The Public Domain rules below apply to most materials (books, songs, photos, art, films, TV shows, to name only a few) that can be copyrighted. Exceptions are audio recordings and unpublished works.
Material published prior to 1923 is now in the Public Domain in the United States.
Material published from 1923 to 1977 is covered by the old copyright law and has a copyright term of 95 years, coming out of copyright on January 1 of the 96th year.
Material published from 1978 forward comes under the new copyright law and has a copyright term of death of the creator plus 70 years, coming out of copyright on January 1 of the 71st year.
If there are several creators (several authors, composers, artists, for instance), it is the death date of the last of them to die that you use to determine when the material will go into the Public Domain.
Audio recordings and unpublished materials have different rules. Talk to an expert.
Works copyrighted by corporations or done as works for hire have a copyright term of 95 years from the date of publication or 120 years from the date of creation, whichever expires sooner.