Maskerade by Terry Pratchett

This book combines Nanny Ogg and Granny Weatherwax's search for a third witch (after Magrat's defection to become Queen of Lancre) with a spoof of the opera generally and The Phantom of the Opera more specifically. I definitely found myself chortling at this book. The witches are some of my favorite Discworld characters, and they don't disappoint here!