The HQ Books are very adaptable to BRP/RQ.
Books such as Storm Tribe/Thunder Rebels/Imperial Lunar Handbooks/Sons of Kargzant/Blood Over Gold/Wintertop's Shadow have a lot of background material with Homelands and Cult writeups.
Homelands are character backgrounds, giving skills and professions available to people from those Homelands. Cult writeups are generally very sketchy with very little background or flavour but describe the spells that a cult gets. I use the HQ format when I am prototyping a cult - I write it up in HQ format then flesh the spells and skills out for RQ.
You have to do a bit of work on the cults to use them in RQ/BRP, but it is not impossible. I'm currently working on adding the later HQ cults to my
HW/HQ Cult Conversions and am planning to convert Homelands as well.
But, there's no reason why you can't take a HQ book and immediately start using it for BRP/RQ.
They are very Gloranthan, though, expect for Mythic Russia, of course. So, you'd have to use them with a lot of adaption outside Glorantha.
But, if you want new cults, new backgrounds and interesting scenarios then I'd recomment the HQ stuff.