Everything you need to understand or teach Witch World by Andre Norton.
Power in all its aspects is a major theme of the Witch World novels. There is the power wielded by the scientific strength of the evil Kolders, but there is also the power of magic that is handled by the witches of Estcarp. Simon, the outlander, is much more familiar with the former, while even the thought of magic makes him uneasy. Yet the author hints that he has his own share of the old magic, due to his Celtic background. And during his most threatening encounter with the head of the Kolder he instinctively employs a magic symbol that he has seen the witches use and is saved by it.
There is plenty of evidence that power corrupts in the insidious rot that enters any country that is touched by the Kolder, and Simon uneasily wonders about the effect that even simple inventions such as the light...