Allow me to indulge in my characteristic hypocrisy and say that, despite its wealth of setting-specific detail, I love Cults of Prax, the first supplement to RuneQuest, written by Steve Perrin and Greg Stafford and published in 1979. This 112-page book details 15 cults and the deities to which they are dedicated, "helpful in clothing a Glorantha-based campaign," according to the introduction. It is quite rightly considered one of the best treatments of religion in a fantasy RPG ever written and it's certainly one of the most inspirational. Every time I read it, I desperately want to start up a campaign in Glorantha, as Cults of Prax ably demonstrates one of the setting's great strengths -- its varied and believable belief systems.
Each of the book's 15 cults is described in some detail, providing information on its history, organization, membership requirements, associated cults, and its unique spells and magic. What's really remarkable is how each entry manages to be so evocative and useful while at the same time being so comparatively spare in its verbiage. Gamers familiar only with contemporary Gloranthan materials might find it hard to imagine but it was once possible to discuss an entire cult in fairly straightforward language without much space being wasted on self-indulgent chrome. (Though, to be fair, each entry does include what can only be called early examples of "gaming fiction," but they're short and often just as useful as the expository text)
The cults themselves are, as I noted, a varied and believable lot, from barbarian deities like Storm Bull to esoteric ones like Humakt to nonhuman ones like Zorak Zoran. Reading through them, one is struck by the fact that none exists just to fill in a spot in a "logical" view of the cosmos, apportioned according to some pre-established schema like alignment. Instead, Cults of Prax presents what might be called a "naturalistic" view of the divine, one that feels more like actual myth, complete with inconsistencies, contradictions, and a disregard for the notion of straight answers. Each cult exists to answer a need within the game setting rather than within the game rules, thereby enabling each entry to give us an invaluable window into Glorantha without having to resort to exhaustive pedantry.
I'm generally down on books like this, which I find tend to forget that they're game products rather than sociological dissertations. Cults of Prax never makes that mistake and yet somehow manages to present religions that feel like more than mere excuses to give characters powerful new spells and magic items. I hate to keep using the word "believable" to describe a book that includes information on the Goddess of the Trolls but it really is apt. Cults of Prax is nothing like Gods, Demigods & Heroes (or its successor, Deities & Demigods), for example, which managed to reduce real world religious/mythological figures to mere game constructs; Cults of Prax does the opposite, elevating game constructs to something approaching credible, if clearly fantastical, belief systems. It's a true classic of the early days of the hobby and a reminder to me that I really was an idiot not to have immersed myself more fully in RuneQuest back in the day.
It's a mistake I'm doing my best to rectify now.