Book excerpt: Terry Pratchett’s fantasy classic Wyrd Sisters, a novel in the Discworld series, is the story of Granny Weatherwax, the most highly regarded non-leader a coven of non-social witches could ever have. Generally, these loners don't get involved in anything, mush less royal intrigue. but then there are those times they can't help it. As Granny Weatherwax is about to discover, though, it's a lot harder to stir up trouble in the castle than some theatrical types would have you think. Even when you've got a few unexpected spells up your sleeve. Granny Weatherwax teams with two other witches — Nanny Ogg and Margat Garlick - as an unlikely alliance to save a prince and restore him to the throne of Lancre, in a tale that borrows — or is it parodies — some of William Shakespeare's best-loved works.