Hello, and welcome!
So, lemme give you a quick overview...
... errrrr... make that a "compressed" overview, but mebbe not so quick...
You want to get the Glorantha Sourcebook. It's pure lore (no game-mechanics), but still very useful for roleplay. It'll give you tons of material whether you game with it, or not.
You may ALSO want to get the (massive, and rather daunting) Guide to Glorantha (2 oversized volumes; roughly equivalent to 4 regular hardcover game-books). It's more of an encyclopedic overview of the entire world; less "lore" and more the simple "facts" of the setting, the "view from 30,000 feet" so to speak. More than any other work, it's the "definitive resource" for Glorantha. Like the Sourcebook, it's free from any game-mechanical bits; and even less apt for the "daily adventurer" sort of perspective. Nonetheless, the ultimate in Gloranthan geekery.
For actual gaming (tabletop, dice and paper) at this point in time -- June 2021 -- you have no less than *FOUR* different RPG's you can buy, each set in Glorantha:
Runequest: Roleplaying in Glorantha -- the flagship of Glorantha
RuneQuest "Classic" edition -- the "golden age" game from the late-70s / early-80s, remastered via Kickstarter
HeroQuest Glorantha -- a more pulpy/heroic RPG, in the new "narrative" tradition of RPGs (i.e. less crunchy-simulationist than RuneQuest). About to become unavailable for legal reasons (sale of the name "HeroQuest").
13th Age in Glorantha -- Glorantha for the D&D-variant "13th Age" RPG.
Any (or all) of these would be good choices... depending more on your own tastes than any intrinsic "better" or "worse" between the games.
We now return you to your previously-announced "not so quick..."
The original Glorantha publications were actually board-games: White Bear, Red Moon was closely followed by Nomad Gods (a 3rd was planned, but never released). These were old-school tactical/strategic "wargames" (played on a hex-map with little counters / chits for the pieces); clash-of-armies stuff, not "adventuring" or role-playing. Nevertheless, they had a remarkable amount of world-building & lore included.
Then Greg Stafford (creator of Glorantha) got an early copy of the brand-new game "D&D." Shortly after that came RuneQuest (RQ), which in turn was shortly followed by RQ 2nd edition (RQ2) ... which was, more or less, a "fixed version" of the first edition (they're very similar, except in a few minor details).
All this was the mid&late 1970s; that, on through the mid-1980s, was the "golden years" of Chaosium & Runequest. The same core game-mechanic, a skill-centric roll-low on d100 (christened "BRP" (Basic Role Playing)) was licensed and used for RPG's for Elfquest, Elric & Stormbringer, Ringworld, and (most famously) Call of Cthulhu, as well as in-house (not a licensed IP) RPGs like "Magic World," and a d20-BRP variation powering King Arther Pendragon.
There followed some decades of ups and downs; of licensing to various companies; of the IP's for "Runequest" & "Glorantha" being separated, and separately-pursued, etc. Gradually, however, Chaosium's fortunes were fading; a sorry fate for a storied company.
Two very-notable events for Gloranthaphiles in this time were:
(1) In 1998, Moon Design Publications was founded by RQ/Glorantha fans. They began by bringing old, mostly-RQ2-era content back into print in compiled form, beginning with a 4-volume "Gloranthan Classics" set, covering 6 major Chaosium works but also drawing from innumerable minor one matching those 4 themed volumes.
(2) In 1999, Greg Stafford (creator of Glorantha) forming a new company (Issaries Inc (Issaries is Glorantha's main god of communication & commerce)) and working with notable game-designer Robin Laws, & MDP (#1 above) to publish an all-new RPG (Hero Wars) for Glorantha. Later editions were re-named HeroQuest, and a setting-free version of the rules was also published for non-Gloranthan of roleplay. This is a totally different RPG, more pulpy and narrative-oriented (than the crunchy & gritty RuneQuest rules).
(But now (2021) the "Heroquest" title has been sold to another company, so in just a few weeks(!) all Chaosium/Glorantha "Heroquest" titles will go away forever -- if you want 'em, buy 'em QUICK!!! HOWEVER, the core game-mechanics and product-line are continuing at Chaosium, re-branded as "QuestWorlds," and there's some hope that the HQ:Glorantha content will return under the new QW brand (eventually).)
Fast-forward to 2015 -- The Return of the Great Old Ones.
Though long uninvolved with operations or management of the company, Greg Stafford & Sandy Petersen (gah, tyvm Jeorg!) retained a controlling interest/ownership in the company. In 2015, they asserted their interest and announced that they would be directly managing Chaosium once again. Right after THAT, they brought aboard most(all?) of the Moon Design team as senior management for Chaosium -- and as part of the ownership team!
Late that year, Chaosium launched a US$30K Kickstarter to "officially" re-print RQ2, and all the old RQ2 supplements. Although many of the supplements were available as "Gloranthan Classics,"
the core rulebook was NOT
the master-files for printing those "Classics" were less than optimal
Chaosium was in serious need of cashflow.
That kickstarter was a smashing success -- almost 7X overfunding (over $US206K on the 30K ask).
This in turn became a big part of their "RuneQuest Next" project: love for the RQ2 ruleset having been financially proven, RQ2 became the core chassis upon which they built the new (current) edition of the game, and easy backward-compatibility to the RQ2 supplements a key design goal. The current edition, "RuneQuest: Roleplaying in Glorantha" (RQG) is -- by far -- the most-Gloranthan core rulebook of ANY edition of RuneQuest.
That brings us up to the modern era, with just one more element worth mentioning -- "13thAiG." Two of the main D&D designers for 3rd & 4th editions (Jonathan Tweet and Rob Heinsoo) had previously created "13th Age" (13A) the d20 rules they WANTED to make, unconstrained by WotC's need to keep it more D&D. They were also longtime Glorantha fans, and licensed the rights to do a 13A version of Glorantha, "13th Age in Glorantha." That kickstarter had launched in 2014, but the "Return of the Great Old Ones" occurred before they fulfilled it; Chaosium got more-directly-involved, and now distributes the game (it was this kickstarter, BTW, that funded (as a Stretch Goal) the acclaimed Glorantha Sourcebook, featured in the TLDR above).
Hope you were taking notes, the Quiz is on Friday (not to be mistaken for @Qizilbashwoman).ndy Petersen