For works published after 1977, the copyright lasts for the life of the author plus 70 years.
However, if the work is a "work for hire" (that is, the work is done in the course of employment or has been specifically commissioned) or is published anonymously or under a pseudonym, the copyright lasts between 95 and 120 years, depending on the date the work is published. For more information on public domain works, see "Using Works in the Public Domain."
All works published in the United States before 1923 are in the public domain -- that is, not protected by copyright law.
Works published after 1922 but before 1978 are protected for 95 years from the date of publication. But if the work was created, but not published, before 1978, the copyright lasts for the life of the author plus 70 years.
If the author died over 70 years ago and the work was never published, copyright terminated on December 31, 2002. If the author died over 70 years ago, and a previously unpublished work was published before December 31, 2002, the copyright will last until December 31, 2047.
For the essential information and forms you need to protect all types of written expression under copyright law, get The Copyright Handbook: What Every Writer Needs to Know, by Stephen Fishman (Nolo), which has been completely updated to provide the latest case law and copyright regulations.