My second ever Discworld novel, this one started out strongly with a lot great humour and some great one-liners, but it sort of petered out for me about mid-way.
The wizard Drum Billet knows that he will soon die and travels to a place where the eighth son of an eighth son is about to be born, but the newborn child is actually a girl, Esk. Billet notices his mistake too late, and the staff passes on to her. But a female wizard is something completely unheard of on the Discworld...
It's a good thing Esk is only 9 years old because if she were any older she'd be too stupid to live, but as a 9 year old, she's just precocious. Granny Weatherwax is... ok, I have to admit I didn't love Granny Weatherwax as much as I expected I would. I think her disdain for basic literary skills turns me off a little bit; I know that makes me sound like a sourpuss, but there it is. I loved her gumption and her pragmatism and I admired her refusal to take crap from anyone. Not to mention her ability to level someone with just her Look.
As I mentioned, the book lost steam for me mid-way; if it had been shorter by about a third I think I'd have enjoyed it more. The whole journey to the Unseen University felt endless to me, although things picked up once Granny and Esk arrive.
Still, Pratchett's weakest book is still a better book than just about most anyone else's best and I'm definitely interested in reading the "better" Discworld books.