The number eight is important to wizards. It is the number of magic in the Discworld; an eighth son of an eighth son is destined to be a wizard.
Wizards are not allowed to have kids. Part of the reason is that an eighth son of an eighth son of an eighth son - and eighth son cubed - is destined to become a sourcerer, a source of magic. Which is a bit of a problem, honestly, because a new source of magic in a stressed out world is highly dangerous. Especially because wizards start getting ideas about who should really be in charge. (Hint: the answer's not only not you, it certainly isn't that wizard over there.)
So when a renegade wizard (eighth son squared) has an eighth son of his own, you just know there's going to be trouble. Add in a possessed staff, an ArchChancellor's hat with its own ghosts, a barbarian hairdresser, a wanna-be barbarian, and an inept poet of a Caliph (with necessary attendant evil Vizier), and you know there's going to be disaster. Someone really heroic is needed to salvage the situation, which is sad since they've only got Rincewind.
If Rincewind were in a movie, he'd be played by someone like Steve Buscemi (all hail Buscemi!) He's a wizard - at least his hat says "Wizzard" - and perhaps the greatest talent in the Discworld at surviving, mostly because he's very good at running away. And, strangely enough, in this book he has the chance to do the right thing, and somehow, he's better suited to the task than anyone else. Maybe because once sourcery gets started, there's nowhere to run away to...
This is the fifth Discworld novel, and one in which Pratchett has a general housecleaning. Sourcery paves the way for ArchChancellor Ridcully, someone who is also very good at surviving, so that the problem of a revolving wizardry cast (through assassination for advancement) is hereby solved. Some characters are still developing, but overall, the humanity of Pratchett's prose shines, and underscores how important personal choice is.