Terry Pratchett 
Wyrd Sisters 
(Discworld Novel 6)

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