We found 2 Reddit comments about Gurps Discworld. Here are the top ones, ranked by their Reddit score.
http://www.amazon.com/gp/aw/d/1556342616?pc_redir=1405525435&robot_redir=1
https://www.amazon.de/Gurps-Discworld-Phil-Masters/dp/1556342616 Sure, its a different system, but it gives a lot of nice background ideas outside of the main books, and its not that hard to transform things to DnD
http://www.amazon.com/gp/aw/d/1556342616?pc_redir=1405525435&robot_redir=1
https://www.amazon.de/Gurps-Discworld-Phil-Masters/dp/1556342616
Sure, its a different system, but it gives a lot of nice background ideas outside of the main books, and its not that hard to transform things to DnD