From the back cover:
Glorantha is a gigantic cube awash in a limiltess ocean, formed by the collision of four forces in primordal eras. It's a world alive with adventure and magic where gods are plentiful and magic real. The realms of power, spirits, gods, and myth are as tangible as the kingdoms of men. It's the home of heroes and monsters, empires and dragons, and of everyday people trying to preserve their way of life.
In Glorantha a hero makes his name in the mortal sphere; those destined to be great heroes travel the perilous paths of the mythical realm. A hero making his own myth can travel to the distant past to aid or oppose the gods in their battles; journey to the underworld and release the souls of the dead; or bend the wildest demons to his will and arness great magical power.
Our aim is to explain the many people, kingdoms, religions, and gods of Glorantha. It's intended to be both a general introduction to the endlessly fascinating fantasy world of Glorantha and a companion book to the Hero Wars role-playing game. We paint the history and present of Glorantha with broad strokes, occasionally highlighting an interesting feature or person.