Maskerade, Terry Pratchett
I've said before how much I particularly love The Witches subset of the Discworld novels, and I think this is possibly my favourite of them all. Granny Weatherwax and Nanny Ogg are particularly resourceful, difficult, funny, enterprising and very determined in their desire to sort out the mess at a big city opera house, when a shadowy Phantom figure is killing people and frightening the living daylights out of many others. They are aided in their quest by a famous opera singer who is well sick of foreign food for reasons that aren't known to many - except the enquiring mind of Nanny of course. The whole story moves towards the final throws of the fat lady having a sing, a manic episode during an ongoing opera and some tricky decisions for Agnes Nit to make. Perhaps should have come with a warning not to listen to this one in the car, couple of times I was laughing so much I had to pull over!
It's Phantom of the Opera, Discworld style, when an aspiring singer and part-time witch wins a part in the big city opera, and quickly learns of a shadowy figure haunting the house. Now her sorceress companions Granny Weatherwax and Nanny Ogg must come to her aid, before the fat lady sings her last rites!