Now as you all know this is one of my favorites and it has the best overview of the Gloranthan monomyth we’ve ever put together, as well as loads of information on the God Learners, the Empire of the Wyrms Friends, the First and Second Councils, etc. Honestly once you have read that chapter, you know the deep background of the setting.
But it also includes essays from Greg and I on how to understand mythology,heroquesting concepts, Glorantha’s cosmology, etc. And a big overview of the pantheons. And a lot more. I reference it more generally than any other book I have as it is just so damn handy.