There is, throughout Terry Pratchett's "Sourcery," a somewhat sorrowful tone being struck amidst the high spirited comedy and dramatic saving the world (again) sorts of action.
Because, at the heart of this story of how a most magical being, one whose very existence could unravel the fabric of the universe, is a scared and cruelly tormented little boy. That Pratchett keeps this in mind, and indeed, makes his salvation just as important as saving the world, is a credit to his skills as a writer and heart as a person.
The eighth son of an eighth son is a wizard. And the eighth son of a normally celibate wizard is a sourcerer, a living font of magical energies. But it's slightly more complicated than that: a father with a grudge against the magical establishment, a magical establishment (the cast of Unseen University, alternately pathetic and for once frightening) that's hungry for power, and a chain of events that quickly gets out of control all hurtle this story towards disaster.
Along the way, we get introduced to the daughter of the greatest hero in Discworld's history, who just desperately wants to be a hairdresser but finds that some things are just in her blood. We also pick up a very unlikely barbarian hero and a literary minded monarch and his fabulous pleasuredome. And, of course, the magic-less wizard Rincewind and his animate Luggage make a triumphant (well, as triumphant as Rincewind gets) return.
"Sourcery" is Pratchett's most heartfelt novel to date, and he puts the characters ahead of plot or jokes -- although there are plenty, especially concerning Conina and Unseen University -- and it makes for one of the best Discworld novels to date.