One of the questions I have seen more than once with regard to the Book of Glorious Joy (well, all right, the only question I've seen more than once) concerns its canonical status. As one questioner put it:
what is the relationship between the Book of Glorious Joy and mainstream Glorantha? Are they letting you define Malkionism in Loskalm?
The simple answer to this question is "no".
But it may deserve a slightly more detailed explanation than that.