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 . . .
”Pratchett”s Discworld books have made millions of people happy” Guardian
”I love Terry Pratchett” Caitlin Moran
Wyrd Sisters is the second book in the Witches series, but you can read the Discworld novels in any order.