Deep secret - Diana Wynne Jones

All over the universe, Magids are at work to maintain the balance of magic, using their own talents to push the right people into doing the right things at the right time. And on Earth, the Magids are working hard to coax the world into its rightful place Ayewards, towards magic.
Rupert Venables has been the junior Magid here for only two years when his sponsor dies; it's up to him to find a replacement. Trouble is, the most promising on his list of five names, Maree Mallory, doesn't want anything to do with Rupert Venables. And while the junior Magid is trying to track down the other four, the fatelines are becoming dangerously entangled on more than one world and magic starts getting out of hand . . .