De Chelonian Mobile (The Turtle Moves) by Didactylos of Ephebe

Title: De Chelonian Mobile 

Author: Didactylos of Ephebe

Publisher: Not known 

Source book: Small Gods by Terry Pratchett 

Not a book but a scroll; De Chelonian Mobile (The Turtle Moves) by Didactylos, contains religious writings from Ephebe. The writer claims that the world travels through the void on the backs of four elephants named Berilia, Tubul, Great T’Phon and Jerakeen, and the elephants stand on the shell of an enormous turtle. The writer even claims to have sailed to an island at the edge of the world and looked over the edge. 

In the eyes of the Omnian religion this is the most terrible heresy because according to Book One of the Septateuch the earth is a ball. Which comes as a bit of a shock to the god Om (currently incarnated as a tortoise) who thinks a sphere is a silly shape for a world. 

But of course, we the readers know that the Discworld does indeed move through the firmament on the back of the giant Star Turtle A’Tuin.

Didactylos, a blind philosopher, isn’t very happy that people believe in the turtle. The turtle just is, he says. He doesn’t think the turtle cares if you believe. It doesn’t give a damn. It’s shocking to him that people in Omnia, who are supposed to believe the world is a sphere, have bothered to copy out his book (in secret) because they believe that the turtle moves.