Magrat Garlick has kind of been promoted to queen of Lancre, which leaves Nanny Ogg and Granny Weatherwax as only a pair of witches. They decide that Agnes "Perdita" Nitt should make up the third in their party, only to find out she's gone to Ankh Morpork to pursue a career in opera. Agnes is an incredibly talented singer (she can sing two voices at once, or project her singing voice half way across the room - and she has actual talent besides). But strange goings-on are afoot at the opera house: someone in a mask is going around murdering people. Nanny and Granny see a disturbing future for Agnes and head to Anhk Morpork to help out (or "meddle," depending on your point of view). Pratchett harps on about the shape of stories, and how they like to play out in the manner they expect.
Perhaps I'm a sucker for pot-shots at Opera - I'm definitely not a fan. Or maybe Pratchett was more on form than the previous book, Interesting Times - which hardly made me laugh at all. There are lots of opera and "Phantom of the Opera" jokes, and a lot of them are quite good. I thought the portrayal of Granny Weatherwax was very good (that's varied between books), and the jokes were quite funny. Not least among these was Nanny's reluctant admission to Granny that she (Granny) has a name among the Trolls, "Aaoograha hoa." Which apparently means "She Who Must Be Avoided." I'm not convinced the conclusion with Agnes was justified, but most of the book played out very well and I enjoyed it considerably.
I love the drawing of Granny Weatherwax below: it's by Paul Kidby, and it's so close to how I pictured her it's shocking. He's managed to capture not only her look but also her force of will and general attitude: not an easy thing to do.