Strata — Terry Pratchett

As a 210-year-old'world-shaper, there isn't much in the universe that surprises or impresses Kin Arad.

At least not until Jago Jalo turns up in her office with an unbelievable tale. Jago, a man presumed dead a thousand years and possessing a cloak of invisibility, deftly lures Kin and two aliens on a journey to a far-distant and totally unique world -- a flat Earth! Together, they hope to discover the secret of this impossible planet and plunder its treasures. But first they must brave dragons, demons, Vikings, witch-hunters, robots, and the countless other perils that await any foolish enough to crash-land on a world that contradicts all the known laws of the universe ...