Nick answers the question, if I have the RuneQuest rules, what should I buy next?If you want to play RuneQuest in Glorantha, I'd recommend buying the books Sun County and/or River of Cradles as a first purchase. Sun County is good because the Sun Dome Templars in Prax have an isolated, xenophobic culture which is fairly easy to get the hang of, but is full of flavour for your characters and adventures (we once characterised them as "uptight Spartans in the Wild West"). River of Cradles includes lots of information about the other peoples of the river valley the Sun Domers live in. Both campaign packs include some ready-to-run adventures, and Shadows on the Borderlands has more still, all set in the same area. After that, it's "Ho for the High Plaines!" with Tales of the Reaching Moon #14 and #15 telling you everything you needed to know about Prax (but were afraid to ask) and including yet more scenarios.
If you like monster-killing, Dorastor: Land of Doom and Lords of Terror make a good set about the indisputable Evil Bad Guys of Glorantha (the creatures of Chaos), with The Book of Drastic Resolutions a neat supplementary source. Dorastor has a good campaign setting for new players (the dangerous frontier settlement of the Risklands), but there's a danger that games here could centre around "getting tough enough to fight the bad guys" without careful referee intervention.
If you want to learn about Glorantha as a world, I'd recommend the boxed sets Glorantha: Genertela (orange box) and Gods of Glorantha (red box) as a starting point. Apart from lots of "factual" material presented from a neutral standpoint, these also have the excellent "What My Father Told Me" and "What The Priest Said" sections: a page or two presenting a culture's worldview (temporal and spiritual) in its own terms. By their nature, these are approachable and player-friendly. I'll also plug our book Wyrms Footprints here: although it includes no RQ stats or rules, it does reprint some of the best Gloranthan articles ever written, Greg Stafford's 50+ page series on the Gods and Goddesses of Glorantha, along with another 60 pages of articles (old and new) illuminating various corners of the world.
My advice would be to avoid the recent, non-RuneQuest publications King of Sartar, The Glorious ReAscent of Yelm and The Entekosiad first time out: these books contain a fair amount of obscure mythical stuff which would be hard to fit into a game, given the lack of supporting material (cult writeups, character generation rules, overview information, etc.). That said, if you can get the Dragon Pass boardgame, Glorantha" and King of Sartar, you'll have a useful set of background information on one of the key Gloranthan settings: we old hands have been resenting the lack of immediately-playable material in Sartar since way back when.
Glorantha is a broad setting (one of the few fantasy worlds where you can play just about anyone, in any kind of story), so there will be a lot of material circulating that may not be immediately useful to your gaming. (Like, I've not even mentioned the Troll books and boxes, etc...).
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