Diana Wynne Jones 
Magicians of Caprona [EPUB ebook] 

Tonino is the only person in the famous Montana household who wasn’t born with an instinct for creating spells, but he has other gifts. His ability to communicate with cats just might help defend the city of Caprona against a mysterious enchanter — but only if Tonino can learn to cooperate with a girl from the hated Petrocchi family of spell-makers.