Procmail How-To Page - Mail Filtering with Procmail. - http://userpages.umbc.edu/~ian/procmail.html
procmail workshop Oct 4, 1998 - Step-by-step guide for retrieving, setting up and configuring procmail. - http://www.stimpy.net/procmail/tutorial/
Debugging Procmail Recipes: Some Tips - Some ideas for what to do when things don't work out as you had expected - http://www.iki.fi/era/mail/procmail-debug.html