What materials are in the “public domain?
For users in the United States, the Google Books Library project treats all their books published prior to 1923 as in the public domain, as well as many publications to which no copyright was attached in the first instance, such as federal, state and local government documents. To be sure there are categories of U.S. material that have entered the public domain, but due to the evolving status covering copyright over the past century, it is difficult to summarize all the possible factors used to determine the status of a particular work.
