Sourcery is a relatively early book in the series, and like all of the books, it more or less stands on its own. Pratchett returns to the life of Rincewind who is probably the most incompetent wizard of all time. When the eighth son of an eighth son breaks the traditions forbidding Wizards to mingle with the fairer sex he has an eighth son himself, bringing a Sourcerer into the world.
So typical things happen – someone tries to cheat death and makes up a destiny for the newborn Sourcerer, setting in place the necessary pieces for the end of the world. Rincewind, a hat, the Librarian, and a handful of other colorful characters do their best to stay alive.
Pratchett can be absolutely brilliant at times and positively unremarkable at others. Unfortunately, Sourcery is one of these other times. There is relatively little outright satire going on, and very few political or pop-culture references, which is where he tends to shine as an author. Aside from a few funny lines and an amusing scene in a pub with the Four Horsemen of the Apocralypse, this entry in the Discworld series is entirely forgettable. On my 10-point rating scale, Sourcery scores 5, an unfortunate score for a writer who can do better.