Wyrd sisters - a novel of Discworld

Granny Weatherwax is the most highly-regarded non-leader a coven of non-social witches could ever have. Generally, these loners don't get involved in anything, much less royal intrigue. But then there are those times they can't help it. As Granny Weatherwax is about to discover, it's a lot harder to stir up trouble in the castle than some theatrical types would have you think.