Cryptography and PGP References

Books

Applied Cryptography, by Bruce Schneier. This has become a standard work for anyone interested in Cryptography or in writing cryptographic software. Mr. Schneier discusses privacy, anonymity, digital cash, voting schemes, etc. and provides source code.
The Codebreakers; The Comprehensive History of Secret Communication from Ancient Times to the Internet, by David Kahn. This is the "Bible" of modern cryptography. Historical rather than technical, it discusses the strengths and weaknesses of cryptosystems throughout history, reinforcing the fact that the "unbreakable cipher" is a myth. Very readable.
Cryptography: Theory and Practice, by Doug Stinson. More mathematically oriented than Schneier's book, Stinson covers modern cryptography in great detail with lots and lots of exercises. This will give you a solid grasp of encryption, but won't particularly help you write cryptographic software.
Cryptonomicon Neal Stephenson's whodunit thriller. Cool crypto plays a big role, including the Solitaire algorithm. Solitaire was specially designed by Bruce Schneier for this book; it's the only algorithm I know of which doesn't require a computer, yet is probably strong enough for actual use.

The sci.crypt FAQ answers Frequently Asked Questions in the newsgroup sci.crypt. This may be the most complete treatment of Cryptography on the net, and is somewhat mathematically intensive at times. This link points to an HTML index of the FAQ, but the FAQ itself is flat text (identical to the periodic posting in sci.crypt).
Snake Oil Warning Signs, the cryptographic snake-oil FAQ. An entertaining read, and educational, too! It discusses the self-promoting claims of folks with a product but no real qualifications, who try to peddle their "eyes-only, super-secret patent-pending" software. This document equips a non-technical reader to spot phonies (say in sci.crypt) without actually understanding the nonsense they spout. A great time saver.
The RSA FAQ maintained by RSA Laboratories. Not surprisingly, this document concentrates on public-key cryptography. It's a little light on technical content, but offers reasonable answers to basic questions.

The The PGP Homepage. These folks apparently own PGP now. Use GnuPG instead.
The International PGP Homepage. This is the authoritative site for international versions of PGP. If you live in the USA and want compatability with older versions of PGP, be aware that there are patent issues when using PGPi in the US.
The MIT Distribution Site for PGP (recently updated) was the original source of PGP, way back when. It no longer offers the original versions of PGP.
Anonymity and privacy on the Internet. This page is pretty up-to-date, and is a fairly standard source of general privacy information.
Fran Litterio's Cryptography page, with excellent references on several topics.

The GnuPG Homepage. Currently GPG will work with PGP 5 and 6 if you set it up right. GPG is much less buggy that UNIX versions of PGP, so I recommend you switch to it if at all possible.
The Linux Encryption Patches Homepage. Here you can find everything you need to run encrypted filesystems on your Linux box. This page is often inaccessible; all you really need is the Loopback Encrypted Filesystems HOWTO and the kernel patches available at ftp://ftp.kerneli.org/
Bruce Schneier's company Counterpane Systems. This is the place to start if you want to study the theory behind modern cryptology.