When do the contents of books become public domain?

A customer has asked us this. Naturally, it depends on the law in the particular country where you are, but in New Zealand the Copyright Act states, "copyright in a literary, dramatic, musical, or artistic work expires at the end of the period of 50 years from the end of the calendar year in which the author dies."

and -

"If the work is of unknown authorship, copyright expires at the end of the period of 50 years from the end of the calendar year in which it is first made available to the public by an authorised act."

So if the author of a book is a natural person, then anything published prior to his or her death becomes public domain 50 years after the year in which he or she died. Thus at present the works of anyone who died prior to 1960 are public domain. As far as we can fathom, if the copyright was owned by a legal entity then the clause relating to "unknown authorship" applies, thus anything published before 1960 (next year it will be 1961, and so on).