A Darkling Plain is set in the distant future, in an age known as the Traction Era. Earth has been reduced to wasteland by a devastating conflict, known as the Sixty Minute War. Nations no longer exist, except in the lands of the Anti-Traction League; Traction Cities - entire cities mounted on caterpillar tracks for mobility - are fiercely independent city-states, using giant jaws to devour one another for resources. Trade is mostly accomplished by airship, though sometimes cities of roughly equal size (unable to devour each other) will stop to trade. Old-Tech (technology from before the Traction Era, some of which is from the 21st century) is the most sought-after commodity.
this book is winner of the 2006 Guardians childrens fiction prize 2006 but that doesn't mean it is only for children
this book is the fourth in the mortal engines quartet
very good book and well worth a pound.