Maskerade

Maskerade is the eighteenth novel in the Discworld series by Terry Pratchett. The witches Granny Weatherwax and Nanny Ogg visit the Ankh-Morpork Opera House to find Agnes Nitt, a girl from Lancre, and get caught up in a story similar to The Phantom of the Opera. The story begins when Agnes Nitt leaves Lancre to begin her career at the Opera House. The Witches leave to obtain the money owed to Nanny Ogg by the publisher of her books, as well as to meddle and bring Agnes Nitt home. Agnes Nitt is being used as a backup voice, giving Christine the spotlight. However, during this time the Opera House is being haunted by a ghost. With the money rightfully coerced out of the publisher Granny Weatherwax pretends to be a rich patron to catch the Opera Ghost. Near the end of the novel a member of the Cable Street Particulars, along with the Witches determine that the finances of the Opera House are deliberately a total mess to disguise the money-laundering going on, and the murders and ... full article at wikipedia