The Wizard of London

The Wizard of London by Mercedes Lackey, 342 pages

This is one of the books in the Elemental Masters series, but all of these books can stand alone. This is the story of two little girls, Sarah, who has been sent to school in London because her parents are missionaries in Africa, and Nan, who is a child of the London streets. The main thing they have in common is that they both have psychic powers of some sort. Despite their many differences, the two girls become good friends and are able to band together when Sarah is threatened by a magical enemy bent on destroying her. With the help of the teachers at their school, they learn more about their powers and try to prepare to defend Sarah from further attacks.