Any book published anywhere before 1923 is in the public domain in the U.S. This is the rule we use most.
U.S. Government publications are in the public domain. This is the rule under which we have published, for example, presidential inauguration speeches.
Books can be released into the public domain by the owners of their copyrights.
Some books published without a copyright notice in the U.S. prior to March 1st, 1989 are in the public domain.
Some books published before 1964, and whose copyright was not renewed, are in the public domain.
If you want to rely on anything except the 1923 rule, things can get complicated, and the rules do change with time. Please refer to our Public Domain and Copyright How-To at <http://www.gutenberg.net/howtos/copyright-howto> for more detailed information.