In short, the Glorantha Source Book is a gaming source book. The Guide is an encyclopedia/atlas/coffee table work of art.
The Glorantha Source Book is intended to complement the Guide to Glorantha as an introductory source book on Glorantha. It covers the basic mythic cycles of the Glorantha monomyth introduces us to the major cultures and basic geography, gives a short history overview, and then jumps right into the greater Dragon Pass area and the Hero Wars circa 1628 - later than the timeline in the Guide and RuneQuest. Of course the Glorantha Source Book will include wonderful new art to complement its mix of familiar and new information.