Maskerade by Terry Pratchett (Discworld 18)

"Dedication: My thanks to the people who showed me that opera was stranger than I could imagine. I can best repay their kindness by not mentioning their names here."

Agnes Nitt, AKA Perdita X Dream, is off to the big twin cities of Ankh-Morpork to join the opera, but the witches back home have other ideas for the young woman, whether she agrees with them or not.