Given what it is "officially" available on the subject, yes, even is there is a certain lack of depth sometimes (to not cannibalize the ElfPak).
So a bit under Mostali & Dragonnewts books.
Several add-on were published in free S&P Mongoose zine. List is here: https://index.rpg.net/display-entry.phtml?mainid=8710
For info, in "Hearts in Gloranha" #2 zine, there are ElfPak Designer Notes:
5 pages by Shannon Appelcline about "The Oak and Thorn" HQ book and the "Elfs: A Guide to the Aldryami" MRQ book.
Beware: these notes are not in the "Hearts in Glorantha Volume 1 issues 1-5 Collected" book.