The GNU Privacy Handbook: Less specific than the gpg(1) man page, more comprehensive than this article.

The GnuPG Home Page: Links to the latest version of GnuPG, GPA and a lot of useful documentation.

The OpenPGP Alliance Home Page:

PGP: Pretty Good Privacy by Simson Garfinkel, O'Reilly & Associates, 1994. Outdated in that it considerably predates GnuPG and OpenPGP, and of course in details of PGP use. Still, however, a comprehensive book about what PGP is, where it came from and how it works.

PGP Security: The company founded by Phil Zimmermann but acquired by Network Associates and for which Phil no longer works. Still the source of “official” PGP software, both commercial and freeware.

Phil Zimmermann's Home Page:

RSA Laboratories' Frequently Asked Questions about Today's Cryptography, Version 4.1 (aka the RSA Crypto FAQ): One of the very best on-line cryptography references/tutorials, particularly as it pertains to public key cryptography.