Maskerade by Terry Pratchett

The Opera House, Ankh-Morpork...

... a huge, rambling building, where masked figures and hooded shadows do wicked deeds in the wings...
... where dying the death on stage is a little bit more than just a metaphor...
... where innocent young sopranos are lured to their destiny by an evil mastermind in a hideously deformed evening dress..

Where...

... there's a couple of old ladies in pointy hats eating peanuts in the gods and looking up at the big chandelier and saying things like: "There's an accident waiting to happen if I ever saw one."

Yes... Granny Weatherwax and Nanny Ogg, the Discworld's greatest witches, are back for an innocent night out at the opera.

So there's going to be trouble (but nevertheless a good evening's entertainment with murders you can really hum...)