Any work created before 1923 is considered to be in the public domain, as is any work created by a United States Government Agency. No copyright is involved.
However, a new edition of a public domain work could be published with an introduction, commentary, illustrations or other additions that WOULD be covered by copyright.
Any work created after 1923, with the exception of publications by U. S. government agencies, may be covered by copyright. For information about copyright for these works, visit the following website: http://www.copyright.cornell.edu/public_domain/