is the 18th book by Terry Pratchett in the Discworld series.
Nanny Ogg was worried. Ever since Magrat left them to get married, there were no longer three witches in Lancre. The maiden, the mother, and the... other one, as it should be. And now Granny Weatherwax seemed to lose contact with the world, drifting away with the animals instead when she had nothing to be angry with. Nanny knew that they needed to find a candidate for the vacancy, before Granny became lost to the world.
Agnes Nitt didn't like being an Agnes. It was such a dull and trustworthy name and not at all exciting and mysterious, like Perdita for example. She had tried calling herself Perdita X Nitt, where X stood for "someone who has a cool and exciting middle initial", but in Lancre that just made people wonder why she wanted to be called Perditax. She wanted to be something more than someone with a good personality. Oh, and good hair too, of course.
All roads lead to Ankh-Morpork as they say and so did Agnes as well, to join the opera. With her (cough) impressive build, and even more impressive voice, she was soon hired as a ghost singer for the real star who had the looks but lacked the voice. But she soon discovers that there is another ghost in the opera house, claiming Box Eight for each premiere night, murdering people and leaving notes behind filled with maniacal laughter...
Obviously, this becomes a job for Nanny Ogg and Mistress Weatherwax. Agnes is not too fond of their involvement, having no wish to go back to the village to become a witch, but this might prove to be too big a mess for her to sort out alone.
As with all of Pratchett's books, Maskerade is full of references, allusions, and parodies. In this case of course the story is based on The Phantom of the Opera but with a serious Pratchettish twist, and, actually, slightly more realistic characters. There is a darker undertone to all his satire, and in this book he questions how people fail to look beyond the surface, be it the mask you wear or the way you look or the role you play. That's what makes his books stand out, apart from being well written and witty and definitely not suitable for reading in public, he still manages to raise very good questions on different aspects of humanity.
All in all, this is a book well worth reading if you're already a Pratchett fan and just missed it, or if you're really into musicals and opera, or if you're just looking for a good time.