Eric by Terry Pratchett is the 9th Discworld book and the 4th to feature Rincewind. In it, young Eric, a resident of the Disc's version of Tenochtitlan, does his best to make a Faustian deal. Except instead of getting the worst of the worst demons, he ends up summoning Rincewind.
Rincewind by dint of not being a demon, is definitely the worst of the worst, but not in a good way. Coming along for the ride is the ever faithful luggage.
Eric ends up being a ridiculous mashup of Faust, Ferris Bueller's Day Off (or similar), and an H. Rider Haggard adventure. It's a short and completely silly novel. Eric is more or less another Rincewind road trip that serves to flesh out the Disc while entertaining the reader.