One of the toughest questions to answer is:

How do I find out if a work is in the public domain?
Or: What is the copyright status of a specific work?

If a work is in the public domain, it means you can use the material (even profit from the material) without seeking permission. If material is still under copyright protection, then anything beyond fair use requires permission and sometimes a payment for that use.

The laws governing copyright have changed so frequently over the years that it's difficult to determine the copyright status of a work. Usually, a search starts at the Library of Congress, here:

http://www.copyright.gov/records/

You can even pay the Library of Congress to conduct a search for you, though their records don't necessarily result in absolute or conclusive evidence.

But there's an outstanding new development from Google Book Search; they've combined and massaged copyright renewal data from multiple sources into one comprehensive document that's freely downloadable. See this blog posting (from Inside Google Book Search) for the full details and the download link.

For anyone working in book publishing, this is huge.