Stanford University Copyright Renewal Database
Books written between 1923-1963 were required to re-register to renew their copyright protection. Some copyright holders did renew, some didn't. The ones who did, had to file a notice which was then published by the US Copyright Office. This is a searchable database of those renewal notices. If you don't find a title here (and it was written between 1923-1963) then copyright likely was not renewed, and it is now in the public domain.