Exapno Mapcase:
I guess that cases of authors dying with absolutely no heirs or estates might be pretty rare compared to the number of cases where people are unable to track down the copyright owner - so I’ll drop that “no heirs” issue.
Exapno is essentially right in all details here, though I’m pretty sure it is an author’s right to declare his own work public domain (it would be odd for the copyright law to restrict an author’s rights to dispose of his work in a manner he deems appropriate). I’ve also seen works where the author granted anyone the right to copy the work freely (sometimes if certain conditions were met), which is essentially public domain.
As usual, whether you’re making money or not is irrelevant to copyright.
If you wish to publish the work, then you must make a good faith effort to discover the copyright holder. If, after a search, you cannot find the copyright holder, you can publish the work with a note of the search and an offer to pay anyone who can show they own the copyright. You’re still liable, but it’d be unlikely that a court would punish you if you made such an effor.
The author ofThe Hundredth Monkey says in his introduction that he wants the book to become public domain, and will never enforce the copyright on it.