What constitutes public domain material is subject to the laws of this country and other countries who are signatory to a number of international agreements about copyright. You can learn of these agreements from the U.S government website Copyright.gov. But for issues that are highly technical in nature you will need an attorney. If you don't have the money for an attorney, I recommend that you drop your interest in a complicated document and go on to something else.
For published material that does not involve a lot of legal complications, I recommend the advice given at Copyright.cornell.edu. This highly useful website has a "Public Domain Chart Updated" that is worth printing out for quick reference.
Of course, "of making of many books there is no end," but I have found The Public Domain: How to Find & Use Copyright-Free Writings, Music, Art & More and The Copyright Handbook: How to Protect & Use Written Works particularly helpful. A simple search of the topic "public domain" at Amazon.com will give you many more.
However, at this point it is important to stress that works in the public domain are only one source for ideas for books that can earn you extra money. My blog site will suggest profitable ideas that have come to me over the years from a variety of sources. Please refer to my blogs regularly for money-making ideas.
