The Chronicles of Chrestomanci omnibus #1 (Chronicles of Chrestomanci, 1-2) [Jones, Diana Wynne]

In this multiple parallel universes of the Twelve Related Worlds, only an enchanter with nine lives is powerful enough to control the rampant misuse of magic--and to hold the title Chrestomanci... The Chants are a family strong in magic, but neither Christopher Chant nor Cat Chant can work even the simplest of spells. Who could have dreamed that both Christopher and Cat were born with nine lives--or that they could lose them so quickly?