If the riddles can be traced to a protected source, they would need to be licensed. If they are entirely your own work, or are sufficiently old enough that they are no longer protected, you can publish the compilation as your own work.
Oddly, because copyright protection is automatic, there's no codified way to publish works into the public domain. You may replace the customary copyright notice with a statement explaining your wish not to assert copyright in the work, or you may find it easier to publish under Creative Commons.
No; the original edition will not enter the public domain until 2032.
If a book is truly out of copyright, it is in the public domain and anyone can republish it in whole or in part.
In place of a copyright notice, explicitly state that you wish to donate the work to the public domain.
Data not in the public domain
The most popular example of software in the public domain is SQLite. There isn't too much software in the public domain, as most of it is licensed under a very permissive free software license, like the BSD license.
A fairly comprehensive list is available at the link below.
if you retained the copyright, yes you can republish it. or if it became public domain then it could be republished.
"Heart and Soul" was composed by Hoagy Carmichael and is still under copyright protection. The song was published in 1938, so it will enter the public domain in the United States in 2034.
One can find pictures of puppies that are considered to be in the public domain on public domain picture collection websites. These are a few of those: Wikimedia, PublicPhoto, Pixabay, PD photo, Photos Public Domain, Picdrome, Alegri Photos, Free Photo Gallery, Public-Domain-Photos, 4Free Photos, Free Stock Image Site, Public Domain Files,Public Domain Depot, Open Galleries, Free Images Collection, Free Digital Photos, Public Domain Photo, Project Public Domain Pictures.
Very possibly Hamlet, but this is a guess. Since Shakespeare's works are public domain, anyone can publish them and there is nobody who will keep track of how many are published.
Union for the Public Domain was created in 1996.
Yes, Shenandoah is public domain.