Fever Crumb (Fever Crumb #1)

Long before the days of Mortal Engines, London is poised on the brink of apocalypse. Huge armoured fortresses are advancing across the wastelands - a new and terrifying kind of enemy. The city is days away from ruin. Buried in London's past is a secret that may save it. But only one key can unlock it - an odd looking orphan named Fever Crumb.