The Secret Keepers - Trenton Lee Stewart, Chicken House

When Reuben discovers an old pocket watch, he soon realizes it holds an incredible power: it can turn you invisible for fifteen minutes. He can't resist the lure of disappearance: for a time, he can vanish from the despotic regime of New Umbra. But the watch's power is even more extraordinary than he imagines. Soon, he's on the run from New Umbra's ruler, The Smoke, who's determined to possess it for himself ...