Pratchett's third book. Some descriptions of this book called it an "absurdist Ringworld, subverting many SF cliches." That's a reasonable short description. This is definitely SF, but you can see him exploring concepts that turned into his "Discworld" series.
[Here's the flap-copy from a hardcover]:
A flat earth? Impossible.
Kin Arad is the 2100-year-old supervisor in charge of resurfacing the newly named planet, Kingdom. When she finds Jago Jalo, a man who has a cloak of invisibility and should have died a thousand years ago, in her office, she decides he must have an unusual tale. He has. He knows where such a world is. It is like the medieval Earth... almost. Leiv Eriksson is setting off for the New World but he will never find it. Instead he sails to the edge of the world and its eternal waterfall. It is obvious that this 'earth' has been built by the Great Spindle Kings, makers of universes, inventors of the strata machine and the ultimate in claustrophobes, and Jalo lures the human Kin, the Kung Marco Farfarer and the fifty-six-syllable-named shand better known as Silver, to undertake a voyage of discovery with him: the rewards must be beyond their dreams... or nightmares.