It might seem a bit ruthless, but it's trivial to search Google for how to determine if a book is in the public domain. Even if the question is about copyright in a country other than the U.S., it's a question that's been asked a thousand times and answered almost as many times on the Internet. For the question to be valid here, IMO, it would need to demonstrate that a satisfactory search had been made to answer the question (downvote rollover: "this question does not show any research effort...").
Without some evidence of an attempt to answer the question themselves, the OP is simply using Stack Exchange as a free research service, which might be the practical truth but really isn't what SE intended.
If that seems too ruthless...
Then I would propose a question with a community wiki answer that lists each book queried on the Stack and the date it entered or will enter the public domain in each country, with the original question not just closed as a duplicate but deleted after directing the OP to the answer on the community wiki.
If the size of that wiki makes you queasy (it does me... worst case, eventually every SciFi/Fantasy book in every country...) it's a bit of evidence that the question should be off-topic.