Tracing its roots to GURPS Fantasy First and Second editions, GURPS Banestorm details the land of Yrth, expanding considerably on what was published before.
From the back cover:
"Welcome to the land of Yrth, a realm of incredibly varied races and monsters – including people snatched from out Earth and other worlds by the cataclysmic Banestorm! Now humans struggle with dwarves, elves, and each other. The Crusades aren't ancient history here – they're current events. And on this world, magic works!"