A Web of Air by Philip Reeve
Two years ago, Fever Crumb escaped the war-torn city of London in a travelling theatre. Now she arrives in the extraordinary crater city of Mayda, where buildings ascend the cliffs on funicular rails, and a mysterious recluse is building a machine that can fly. Fever is the engineer he needs - but ruthless enemies will kill to possess their secrets. The fabulous sixth book in the Mortal Engines series, from the brilliant and award-winning Philip Reeve.