If you're interested in Greasemonkey, you'll be interested in this: Dive into Greasemonkey, another fine publication by Mark Pilgrim, whose enthusiasm Greasemonkey has benefitted from greatly.
Dive into Greasemonkey is
It's also excellent, and includes the latest features. If you'd like to know something about coding for Greasemonkey it's there. If you'd like to evangelize Greasemonkey to someone else, it's there. If you'd like to learn how to use Greasemonkey, it's there.