Maskerade by Terry Pratchett

The Opera House, Ankh-Morpork . . . a huge, rambling building, where innocent young sopranos are lured to their destiny by a strangely-familiar eveil mastermind in a hideously-deformed evening dress . . .

At least, he hopes so. But Granny Weatherwax, Discworld’s most famous witch, is in the audience. And she doesn’t hold with that sort of thing.

So there’s going to be trouble (but nevertheless a good evenin’s entertainment with murders you can really hum…)