Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd Edition by Bruce Schneier. John Wiley & Sons, 1995.
Dice-Based Password Generation Method: diceware.com
The GNU Privacy Handbook: gnupg.org/gph/en/manual.html Less specific than the gpg(1) man page, more comprehensive than this article.
The Official GnuPG Home Page: gnupg.org Has links to the latest version of GnuPG, GPA and a lot of other useful information.