Unsure whether a work is protected under copyright or is available in the public domain?
Check out the nifty slider in which tells you the copyright status by year and whether permission is needed. The slider is published by Michael Brewer & the ALA Office for Information Technology Policy
Cornell University offers more detail in its table, Copyright Term and the Public Domain in the United States. It is organized by Type of Work and Copyright Term.
Hat tip to my colleague Nancy Paul and Luther College.