If you think your commercial enterprise system is complex, place yourself inside an operation command and control system for warfighters, and it will make your problem look like HelloWorld. Such offensive and defensive military systems are typically hard or near real-time (rebooting in the middle of a fire fight is a wee bit embarrassing), cannot fail (otherwise people will die or opportunities to get the bad guys will be lost), exhibit bursty behavior (war is generally characterized by long periods of boredom interspersed with brief episodes of intense panic), are not so much applications as they are systems of systems (with lots of legacy and lots of impedance mismatches between systems within which one must stuff people), are warped by historical, organizational, national, and geo-political forces (imagine having a Senator on your development staff), and are constantly changing (yet individually have incredible inertia and thus resistance to change).
I've got a few such systems in mind for the Handbook, the challenge for me being that some of the really interesting ones are classified, although it's still amazing what some solid detective work will uncover from public documents.
One of the more important architectural standards in this space is DoDAF, the DoD Architecture Framework, a successor to C4ISR, the Command, Control, Communications, Computers, Intelligence, Surveillance, and Reconnaissance architecture framework. DoDAF recommends 26 different architectural workproducts, organized into four architectural views (operational view, systems view, technical standards view, and all view). Having worked in the DoD space, I absolutely respect the standards work people are trying to do here - this is a really hairy problem - but, my personal opinion (and not necessarily that of my employer or of any other person, living or dead) is that the DoDAF (and its predecessors), while well-intentioned, encourages the production of deeply engineered and disparate work products that give the illusion that one is doing systems architecture.