I'm trying to find out what type of legal form is required
to get permission to publish responses to prank letters.

By prank letters I mean the humourous type of letters like in the
books of Henry Root (UK) or Ted L.Nancy (Seinfield- US).

What type of legal document would this be called?
The legal form would be to get an individuals permission to re-print
their response to such a letter - either on a website or in book form.
This is also needed because in the case of companies - company logos would be 
re-produced.


Ryan