Half-witch : a novel / John Schoffstall.

Enhanced descriptions from Syndetics:

Set in a fantasy world of hungry goblins, powerful witches and human criminals, Half-Witch follows the unlikely friendship between Lizbet and the unpleasant sarcastic witch girl Strix on a twisted journey to rescue her father from prison.

In the world in which Lizbet Lenz lives, the sun still goes around the earth, God speaks directly to his worshippers, goblins haunt every cellar and witches lurk in the forests. Disaster strikes when Lizbet's father Gerhard, a charming scoundrel, is thrown into a dungeon by the tyrant Hengest Wolftrow.

To free him, Lizbet must cross the Montagnes du Monde, globe-girdling mountains that reach to the sky, a journey no one has ever survived, and retrieve a mysterious book. Lizbet is desperate, and the only one who can help her is the unbearable witch girl Strix.

As the two girls journey through the mountains and into the lands of wonder beyond, Lizbet discovers--to her horror--that Strix's magic is turning Lizbet into a witch, too. All while, a revolution in Heaven is brewing!