According to LSpace’s Discworld reading order guide (v2.21), Sourcery is the third book in the Rincewind Novels thread of Discworld books.
Pratchat Podcast (@PratchatPodcast) December 09, 2017
Terry Pratchett – Sourcery (1988)
( Rincewind, #03; Discworld, #05 ) 🎧
A sourcerer is born in Discworld: a wizard so powerful that he makes all other magicians look like a bunch of fools in pointy hats. Now, suddenly, Discworld is brought to the brink of an all-out thaumaturgical war. The only hope for peace is Rincewind, the failed magician who has a risky plan to save the world. He enlists the help of several odd new characters, including Conina the barbarian hairdresser, Nijel the Destroyer, and a yuppie genie who sees lamps as a growth industry.
“There was a man and he had eight sons. Apart from that, he was nothing more than a comma on the page of History. It’s sad, but that’s all you can say about some people.”
Rincewind is a failed student at the Unseen University for wizards in Ankh-Morpork, and is often described by scholars as “the magical equivalent to the number zero”. He spends most of his time running away from bands of people who want to kill him for various reasons. The fact that he’s still alive and running is explained in that, although he was born with a wizard’s spirit, he has the body of a long-distance sprinter. Rincewind is also renowned for being able to solve minor problems by turning them into major disasters. His unique “skill” is implied to be due to being the chosen one of “The Lady”, the anthropomorphic personification of luck (both good and bad).
“There are eight levels of wizardry on the Disc; after sixteen years Rincewind has failed to achieve even level one. In fact it is considered opinion of some of his tutors that he is incapable even of achieving level zero, which most normal people are born at; to put it another way, it has been suggested that when Rincewind dies the average occult ability of the human race will actually go up by a fraction.”