Wyrd Sisters (Discworld book 6), Terry Pratchett

Destiny is important, see, but people go wrong when they think it controls them. It's the other way around.'Three witches gathered on a lonely heath.

A king cruelly murdered, his throne usurped by his ambitious cousin. A child heir and the royal crown, both missing. Witches don't have these kinds of leadership problems themselves - in fact, they don't have leaders.

Granny Weatherwax is the most highly regarded of the leaders they don't have. But even she finds that meddling in royal politics is a lot more complicated than certain playwrights would have you believe. Particularly when the blood on your hands just won't wash off .

. . Wyrd Sisters is the second book in the Witches series, but you can read the Discworld novels in any order.