[Legbreaker]

A source book would probably need to focus on the southern part of the country plus a bit of SE Asia (Vietnam, Cambodia, etc but excluding the island nations such as the Philippines and Indonesia) to fill the pages with just a cursory coverage of the north.
Much of what's already in Bangkok, Merc and the Gazetteer would be applicable with a few minor tweaks here and there perhaps (Bangkok's vague enough that it can be used in either timeline without any trouble).
Northern China is just one big Devastated region with scattered random encounters and the odd Cantonment. Civilians have probably all cleared out of died long ago, and the soldiers that are left would probably be suffering quite badly from radiation, starvation and disease. The Pact units are likely looking to move back north out of the devastated zone, or east into Korea if they're still paying some sort of lip service to higher command.
Chinese units have likely become little different to marauders preying on each other and the handful of civilians too stubborn or unable to leave. Give it another couple of years and the whole region will likely be deserted for the most part with only rare settlements spread far apart to be found.