Chris Priestley & Philip Reeve 
Traction City: World Book Day 2011 [EPUB ebook] 

In a dangerous future world where gigantic, traction-powered cities attack and devour each other, London hunts where no other predator dares. Hidden in its vast superstructure is a murderous creature that severs the right hands of its victims. A rebellious young aviatrix and a secretive scavenger boy are about to come face to face with a robotic Stalker that is terrifyingly out of control. This story for older readers was published for World Book Day 2011, and is now available in electronic format for 79p.