Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd Edition by Bruce Schneier. John Wiley & Sons, 1995.

Dice-Based Password Generation Method:

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

The Official GnuPG Home Page: Has links to the latest version of GnuPG, GPA and a lot of other useful information.