Here are a couple of thoughts from an IR perspective:
Case 1: 50% population loss, no infrastructure damage, no climate change (e.g. a limited pandemic)
The nation-state system is likely to remain intact, aside from a few states that have been totally depopulated.
A fundamental reorganization of the global balance-of-power (might lead to the total demise of current superpowers like the US and China), which would cause a collapse of the international order (no more UN, no more meaningful alliance arrangements).
Thanks for the recommendations! Glad to see that you've compiled an IR reading list related to EA causes.