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 and confirmed as a good source of information. However, it has not been revised since its original publication in 1992. As such it only covers X11R4 and X11R5, anything specific to X11R6 will not be covered.
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.