GURPS Discworld Designer's Notes - GURPS 3e version (Pyramid #29)
Worlds of DiscworldEdit
Discworld is not formally part of the Infinite Worlds, but there is no reason it couldn't be. In addition, there are multiple discs.
Dungeon Dimensions (pg 269)
Alternate Discworld (pg 269)
The 3rd edition version of this book and its supplement are out of print and not available as a PDF.