The Real Boy by Anne Ursu
Oscar knows he's different. He has trouble relating to the people that come to the wizard shop, magician's apprentice, Wolf, and to his master, the Caleb. He is more than content to be the shop boy, spending his time gathering and mixing the herbs that Caleb sells to people who want to buy his magic and talking to the cats that roam the house. But when Wolf is killed, Caleb disappears, and a beast appears intent on destroying anything magic, Oscar has no choice but to try to save the children of the city and find out how to stop the beast from destroying his world.