We give only a few sources here. Consult the first book and selected entries in its bibliography for an introduction to cryptography.
There are many on-line sources of material on cryptography, PGP and data security, but most of them are quite ephemeral. The Frequently Asked Questions posted periodically to the alt.security.pgp and sci.crypt Usenet newsgroups are informative and provide up to date references to on-line material. Information about PGP and related matters can be found at the URL http://www.pgp.net/pgpnet/ and links therein. PGP distributions, documentation, integration tools and such like are available by anonymous ftp from ftp.pgp.net in sub-directories of /pub/pgp --- get README.html in the first instance.
Schneier has written the definitive reference work for modern computer cryptography. Several chapters give an introduction to the field; many more go into details of cryptographic algorithms, protocols and implementations. It is especially valuable for its comprehensive bibliography with over 1500 pointers into the literature.
Bruce Schneier, Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd edition, Wiley, 1996, ISBN 0-471-11709-9
A general survey of secure electronic mail, including descriptions of both PEM and PGP is given by:
Bruce Schneier, E-Mail Security: How to Keep Your Electronic Mail Private, Wiley Publishing, 1995, ISBN 0-471-05318-X
Three books concentrating on PGP in particular. We prefer Garfinkel's version, but all are good treatments of the program and its use.
Simson Garfinkel, PGP: Pretty Good Privacy, O'Reilly & Associates, 1994, ISBN 1-56592-098-8William Stallings, Protect Your Privacy: The PGP User's Guide, Prentice Hall, 1994, ISBN 0-13-185596-4
Philip R. Zimmermann, Official PGP User's Guide, MIT Press, 1995, ISBN 0-262-74017-6