Deerheart by Eoin Colfer

A gripping and suspenseful ecological fairy tale set in old Ireland, from two legendary Irish children's book-makers.

Three fairy shapeshifter witches are the guardians of the Creedon Stone, the source of all magic, which runs all the way from the middle of the forest to the centre of mother earth. The witches' changeling powers allow the eldest sister, Scrawb, to fight as a crow; the middle sister, Hackle, a wolf; and the youngest, Deerheart, a speckled red doe. When humans come to steal the magic for their own ends, young Deerheart must step up and discover the power within her.