According to recent European Commision (EC) report there at least 3 million orphan books in Europe. Oprhan works are works which are still in copyright but rigth holders cannot be traced. EC gathered responses from 22 cultural institutions which are involved in digitisation work, including national libraries, archives, and national films collections. From Poland the study participants were the National Library and the National Digital Archive.
Problem with clearing information about right holders is not limited to books, there is a huge number of photographs and audiovisual materials whose copyright owners are unknown or untraceable.
The European Commision report can be downloaded here.[source]