The Saga of Grittel Sundotha, by Ardath Mayhar (epub/Kindle/pdf)
Seven-foot-tall Grittel, daughter of the Lord of Sundoth, has refused the impossible marriage arranged by her domineering mother. Now an embarrassment to her family, she is sent forth to make her own way in the world. But Grittel Sundotha is no shrinking maiden. Along the way she castrates a noble noted for his lechery; defeats the assassins he sends after her; and, having been trapped by a sorceress in an enchanted forest, winds up boiling the witch in her own cauldron, taking with her the spell book she finds there. This proves invaluable, for she finds that she has an innate gift for sorcery.
When she meets a space ship's crew whose vessel has been pulled from their own dimension (where magic is impossible), into Grittel's (where technology does not work), she attempts to prove her abilities by "clearing" the Captain's head--and instead, inadvertently makes it invisible. She knows that she must find the wizard whose spell has trapped the ship, for only he can send the vessel home.
But not all mages on this world practice white magic!