Koom Valley Codex
This book plays quite a role during the events of Thud!. It was written about Methodia Rascal's masterpiece, the Battle of Koom Valley, detailing a secret. People had read it for years without understanding until Helmclever took a copy home, where it was read by Grag Hamcrusher, who then turned up in Ankh-Morpork and went about finding the Device it referred to.
Refers to the Da Vinci Code, a publishing sensation from 2003, detailing a spurious chase through many legends of antiquity involving a treasure that the average public could not comprehend even if they had hold of it.