Lily Grim and The City of Undone by Andy Ruffell

Lily Grim's life is a mystery. She has lived with her guardian Gabriel, in their rundown second-hand shop in the City of Undone, for as long as she can remember. For years, the city-dwellers have lived in uneasy discord with The Others - a community of wanderers forced to shelter within the city walls, after they were driven from their nomadic camps in the wilderness.