Simson Garfinkel wrote a book about PGP (O'Reilly and Associates), and MIT Press has published Zimmermann's documentation for several PGP versions, as well as the source code for them in separate volumes. PGP long included Zimmermann's documentation in every copy.
http://www.pgpi.org - Ståle Schumacher's Web site. Information on currently available open source versions of PGP, including 2.x versions, and information on GPG and PGP generally.
