A Wizard's Guide to Defensive Baking by T. Kingfisher

Fourteen-year-old Mona isn't like the wizards charged with defending the city. She can't control lightning or speak to water. Her familiar is a sourdough starter and her magic only works on bread. She has a comfortable life in her aunt's bakery making gingerbread men dance. But Mona's life is turned upside down when she finds a dead body on the bakery floor. An assassin is stalking the streets of Mona's city, preying on magic folk, and it appears that Mona is his next target. And in an embattled city suddenly bereft of wizards, the assassin may be the least of Mona's worries ...