Linux in a Nutshell, 3rd Edition by Ellen Siever, Stephen Spainhour, Jessica P. Hekman and Stephen Figgins. Sebastopol, California: O'Reilly & Associates, 2000. An extremely useful book that includes a complete reference for the iptables command.

The Home Page for Netfilter (iptables):

The Home of rsync: One of my favorite file-transfer tools, which works particularly well over tunnels.

The Home Page for Stunnel, a Versatile and Strong Tunneling Utility: