Mortal Engines by Philip Reeve

London is on the rampage again, pursuing a small mining town across the dried-up bed of the North Sea. It's a quasi-Victorian futuristic world where cities prey on each other, recycle their parts and capture their inhabitants. Tom embarks on a journey to return to the city that has abandoned him, accompanied by the cold and damaged Hester. Shortlisted for the Whitbread Children's Book of the Year.