Steve Perrin and Greg Stafford’s 1979 booklet Cults of Prax is a source book for the venerable RuneQuest roleplaying game. It was, I believe, the second publication in the RuneQuest line. Cults of Prax outlines the religions found in Prax, a wasteland adjacent to the Dragon Pass region featured in the original rule set. This booklet fleshed out the backstory to the world of Glorantha, the setting used for most (but not all) editions of RuneQuest.
The somewhat dry material was spiced up with comments from a wandering trader with long experience of the peoples of the Prax region.