Here are some helpful guides to investigating the copyright and renewal status of published or unpublished works.
Samuel Demas and Jennie L. Brogdon, "Determining Copyright Status for Preservation and Access: Defining Reasonable Effort," Library Resources and Technical Services 41:4 (October, 1997): 323-334.
How to investigate the copyright status of a work. Circular 22 from the Library of Congress Copyright Office which provides general information on what is and is not protected by copyright, how to search for copyright records and updates on legal aspects of copyright issues.
Copyright Term and the Public Domain in the United States is a chart indicating when a work will go into the public domain whether published or unpublished in the U.S. or outside the U.S.
Copyright Registration for Online Resources This circular also from the Library of Congress and gives information about copyright registration of online works made available over a communications network such as the Internet.
Copyright Research from Stanford University gives instructions for finding information about ownership and copyright validity.
Copyright Clearance Center can verify the copyright status of a work for you because they manage the rights relating to over 1.75 million works and represents more than 9,600 publishers and hundreds of thousands of authors and other creators, directly or through their representatives.
Harry Ransom Center contains the freely-accessible WATCH File (Writers, Artists and their Copyright Holders), which gives searchers information on contact persons for copyright holders for U.S. and U.K. authors and artists.