APA (7th ed.) Citation

Zimmermann, P. (1996). The official PGP user's guide. MIT.

Chicago Style (17th ed.) Citation

Zimmermann, Philip. The Official PGP User's Guide. Cambridge (Massachusetts) [etc.]: MIT, 1996.

MLA (9th ed.) Citation

Zimmermann, Philip. The Official PGP User's Guide. MIT, 1996.