This is the 18th 'Discworld' book by Terry Pratchett, and one I thoroughly enjoyed both on first reading about seven years ago, and more recently. It's a novel that features the witches, primarily Agnes Nitt, a rather well-built girl from Lancre who wants to be a singer. She travels to Ankh Morpork to join the opera, and is followed shortly afterwards by Granny Weatherwax and Nanny Ogg, who hope to persuade her to return.