I am, along with a number of other folks, using Codex sourcebooks to inform our Mythras gaming. Mythras is a d100 ruleset based on Runequest (it’s basically Runequest 6th Ed). Several of us are rather into the 1400s-1600s and the Baltic sourcebooks align with that era as well as a Mythras book called Mythic Constantinople (which covers the period of the Conquest).