Mortal Engines
The Hungry City Chronicles
by Philip ReeveTom Natsworthy, third class apprentice historian, finds out that his hero Thaddeus Valentine isn't really such a great guy the day that that London finally finds a smaller town to eat. Among the town's inhabitants is a girl named Hester Shaw. She runs for and tries to kill Valentine, but Tom stops her. Tom runs after Hester Shaw and confronts her. She tells him her name and jumps out the waste chute, into outland. When Tom asks Valentine who Hester Shaw is, Valentine pushes Tom down the waste chute to join Hester Shaw. Tom and Hester have lots of dangerous adventures, in each escaping death only through astounding luck. We are also told what is going on in London, through the eyes on Katherine Valentine, our villain's teenage daughter.
In case you're a little apprehensive about reading a part of a series, not to worry, this is the first one in the series (the author's first book, even), and it does still have a fairly final sort of ending, although it is the right sort for a series. The premise of the series, that, in the 40 somethingth century, we, the ancients will have mostly destroyed the world, and technology will depend mostly on what we developed, is a little weak, but still interesting. Just where this world stands technologically is a little unclear. On the one hand, heavier than air flying machines are discussed as though they don't exist, but airplanes are almost certainly extant(extant means currently existing, Nate) in the story.
It also seems bizarre that "seedy"s would have made it that long, without the continued existence of the computer.
Happy reading.