Reeve's first book for older readers was "Mortal Engines", which won the Nestlé Smarties Book Prize in age category 9–11 years and made the Whitbread Book Award shortlist. "Mortal Engines" is the first book in a series sometimes called the Mortal Engines Quartet (2001–2006), which includes "Predator's Gold", "Infernal Devices" and "A Darkling Plain". The books feature two young adventurers, Tom Natsworthy and Hester Shaw, living in a lawless post-apocalyptic world inhabited by moving cities. For the fourth volume, Reeve won the once-in-a-lifetime 2006 Guardian Children's Fiction Prize, judged by a panel of British children's writers.