Traction City: World Book Day 2011 by Philip Reeve

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.