The Wall And The Wing By Laura Ruby

A few, nicknamed leadfeet, are forced to forever spend their lives closer to the ground. But one night, a girl named Gurl—a leadfoot, an orphan, a nobody—discovers that she can do something much better than fly. She can become invisible. Along with a new friend, a boy named Bug, Gurl begins a quest that takes her on a wild ride through the magical city, all the way to the handsome but lethal Sweetcheeks Grabowski—the gangster who holds the key to Gurl's past . . . and the world's future.