Pratchett Terry. Discworld 18. Maskerade

There are strange goings-on at the Opera House in Ankh-Morpork. A ghost in a white mask is murdering, well, quite a lot of people, and two witches (it really isn't wise to call them "meddling, interfering old baggages"), or perhaps three, take a hand in unraveling the mystery. The witches 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. Fans of the popular Discworld will be happy to see some old friends again in Maskerade, the 18th novel in the series.