I read a trilogy years ago in High School that basically took place many, many years after a war of some sort (I recall nuclear). The trilogy opens with a native-type that basically investigates a crash site of the "gods" and realizes they're human. He's adopted into this society that scavenges old cities for supplies and gear.
We come to find out that the society was created by a bunch of really wealthy people who saw this coming. They created several isolated sites and left a series of gates to be opened by riddle that led them, through the course of the trilogy, to other sites like their own.
Ultimately, they find the last gate and, in the process of unlocking it, destroy themselves.
I recall one of the books being called Yggdrasil or something similar. It's really perplexing because I can't find any mention of it anywhere but I loved the trilogy and want to reread it again.