Government documents and works published before 1923 will be in the public domain. Works that were published between 1923 and 1977 without notice of copyright or works published between 1923 and 1963 for which the copyright was not renewed will be in the public domain. See this site by Lolly Gasaway for details. http://www.unc.edu/~unclng/public-d.htm. When in doubt, contact the publisher.