Estimated 25-50 million books are orphaned works, and their copyright holder may step forward at any time after you've treated it as unlicensed, it's perpetually uncertain (but showing due diligence in finding and contacting the rights holder is considered adequate).
For US works published after 1977 and most works between 1898-1945 the US copyright office has a database:
https://wikipedia.org/wiki/Orphan_works_in_the_United_States
https://martincopenhaver.github.io/files/orphanworks.pdf [PDF]
Estimated 25-50 million books are orphaned works, and their copyright holder may step forward at any time after you've treated it as unlicensed, it's perpetually uncertain (but showing due diligence in finding and contacting the rights holder is considered adequate).
For US works published after 1977 and most works between 1898-1945 the US copyright office has a database:
https://publicrecords.copyright.gov/
but I don't know a good catalog for non-US.