If you are coming new to RuneQuest and Glorantha with RQG, it has never been easier to get the lore, the cults, and get gaming. Grab the current RQG materials, supplement it with stuff you find interesting from the fan publications with the Jonstown Compendium and if you still need more, you can get RQ2 materials from Chaosium that are 90% compatible with RQG. If you want to deep dive into lore there is the Glorantha Sourcebook and the Guide to Glorantha. Go nuts!
For those of us that have been playing in Glorantha for a long time, headaches can come if you try to impose the Hero Wars (HW) materials of the early 2000s. Now that was written at a different time, represented a very different direction from where Greg and I took things with the Guide to Glorantha and setting up the new RQ line. It was also written for a completely different rules system (and one that Greg really never understood). As far the creative team is concerned, the HW materials do not provide any information not already in the RQG materials (which is also our advice to anyone wanting to write for Chaosium).
My advice is just to take the RQG materials and not try to supplement it with HW materials or HW-related fan theories. Just start fresh with the stuff available through Chaosium. Or if you really need to, supplement it with the RQ2 materials. Heck, that’s available through Chaosium as well.
But if you want to use the HW materials, YGWV. Heck if you want to supplement Glorantha with Tomb of Horrors or Ars Magica, YGWV. Just please don’t use that to gatekeep others or complain that RQG doesn’t incorporate such materials.
Everything canon in those books in the HQ Era books, namely Sartar KoH and the Sartar Companion are already in other identified sources. And once the Sartar Book (it is in the next wave of books for 2024( comes out there will finally be a real RQ sourcebook for Sartar.
What about RQ3? Its far closer to RQG. RQ3 is also easily compatible with RQG – the main difference is that hit points are calculated a little differently in RQG (meaning that a duck player character in RQG will generally have more hit points in RQG than RQ3 and a giant will generally have less hit points in RQG than RQ3).