The Procmail Companion is one of those rare books that combines here-is-how-to-do-it for beginners with a complete reference for those who already know how to do some of it.

The book starts with a description of procmail, how to install it, how to intelligently set up a .procmailrc with logging, and how to debug. It then moves on to simple recipes (configurations), walking through in exquisite, useful detail what each command does.

The book covers, among other things, the procmail regular expression syntax (a little different than Perl or grep), scoring, variables, connecting with external programs. It has a full section describing how to manage a mailing list, a cookbook of very useful scripts, and a full reference.