They have spent time and money finding the images and getting them into a useable format for computers. This is what you pay for and most likely DOVER has copyrights on the digitized art work, not the original.
There is no central repository or list for works on which copyright protection has expired.
After copyright term has expired, materials enter the public domain.
I don't think you can see it anywhere online legally. Remember, this is a commercial work, whose copyright has not expired.
Once copyright term expires, the material enters the public domain.
Only if the copyright has expired.
Yes, Shenandoah is in the public domain. This traditional American folk song dates back to the 19th century and its copyright has expired, making it part of the public domain.
"Out of print" does not equal public domain. A work is safe to use only if the copyright has expired or deliberately been placed in the public domain by the rights holder. Or if the copyright has expired and was not properly renewed.
A work that is not subject to copyright protection would be in the public domain. Examples are works of the federal government (such as NASA imagery) or works for which protection has expired.
No; protection is expired, and the works are in the public domain.
Works for which the term of protection has expired are in the public domain.
No; any protection has expired.
Most of them are old enough that copyright term would have expired, but newer translations, adaptations, and illustrations may still be protected.