"An Unhurried View of Copyright", Benjamin Kaplan, Columbia University Press, 1967, pp. 4-5.
"Five Hundred Years of Printing" pp. 218-230, S. H. Steinberg, Penguin Books, 1955, revised 1961.
"The Street Performer Protocol and Digital Copyrights" by John Kelsey and Bruce Schneier, at http://www.firstmonday.dk/issues/issue4_6/kelsey/.