Check your local laws if you're not in the United States, though. Some countries don't acknowledge American copyrights at all, others don't have a concept of public domain. In the latter case, the principle is that, if there's any value in a concept, then someone, somewhere, must own it.
If you're in the United States, though, or if your country applies a "shortest term" rule (that is, it's public domain if it's out of copyright in your country OR the country of origination), then you should be OK.
Lastly, if you have concerns about the legality, definitely double-check the copyrights on your own. Finding something here, at Project Gutenberg, on the Internet Archive, or wherever else doesn't carry any legal weight, whereas documenting your own search will be helpful.
