Elemental Masters, book 5: The Wizard of London
Mercedes Lackey2005

The Harton School for Boys and Girls, run by Isabelle and Frederick Harton, is one of the few schools that takes students whose magic doesn't pertain to the elements, and who are, therefore, frequently ignored by the Elemental Masters. Such unheeded gifts include clairvoyance, telepathy, and the very rare ability to truly communicate with the dead. Sarah Jane's parents, missionary healers in Africa, send the 12-year-old to Harton, and she is happy there, especially after she befriends Nan, a street urchin. After an attempt is made on Sarah and Nan's lives, it is clear that a powerful Elemental Master wants one or both girls dead. Isabelle Harton must seek the aid of the Elemental Masters of London, though the Masters' Circle is led by Lord Alderscroft, who once cruelly jilted her.