A related document on WWW is ``What to do when Tk says that your display is insecure'', http://ce-toolkit.crd.ge.com/tkxauth/. It was written by Kevin Kenny. It suggests a similar solution to X authentication to that in this document (xauth). However, Kevin aims more at using xdm to steer xauth for you.
The X System Window System Vol. 8 X ``Window System Administrator's Guide'' from O'Reilly and Associates has also been brought to my attention as a good source of information. Unfortunately, I've not been able to check it out.
Yet another document much like the one you're reading now, titled ``Securing X Windows'', is available at http://ciac.llnl.gov/ciac/documents/ciac2316.html.
Also check out usenet newsgroups, such as comp.windows.x, comp.os.linux.x, and comp.os.linux.networking.