I just read Fever Crumb, which I didn’t realize was a prequel to Reeve’s Hungry City Chronicles (Mortal Engines et al). It starts out calmly enough, in a future London after “The Downsizing” where old-tech (our modern technology) is the hot commodity. Fever Crumb is an orphan raised by Engineer Dr. Crumb to be rational and emotionless. She leaves home on assignment and is immediately tracked by an old hunter who thinks she is one of London’s not-quite-human former overlords. Her new employer thinks she holds the key to a treasure vault of old-tech, and his efforts to jog her memory unleash a torrent of memories and emotions that Fever is completely unprepared for. As she begins to investigate the mystery of her origins, London erupts into chaos, and nomadic invaders approach London in huge moving traction castles…
Fever Crumb is the start of a four-book series of its own that chronicles the origins of the Municipal Darwinism and traction cities of the first series. Reeve’s steam-punk, post-apocalyptic books are filled with colorful characters, implausible but brilliantly realized settings, and roller-coaster plots.
Mortal Engines is a steamroller of a book, chronicling the adventures of Tom Natsworthy, an apprentice Historian in London, and Hester Shaw, a horribly scarred girl who is trying to kill Thaddeus Valentine for murdering her parents. After Valentine throws them off of London, they pursue it over the wasteland that was London, but they are captured many times and also pursued by the Stalker Grike, a reanimated metal warrior who raised Hester and now has some strange plan for her…
This is a series I HAD to finish (and you KNOW I don’t often let myself do that!) It’s hard to book talk it because so much happens and there are so many characters. I’m hoping Peter Jackson has more success adapting it to the silver screen. (Wikipedia says he is making a movie of it, so it MUST be true! 
I do NOT know why the American editors changed the name of the stalker from Shrike to Grike. (Maybe b/c it’s the monster from Dan Simmons’s Hyperion.) Shrike sounds cooler.
Younger readers will enjoy his straight-up steampunk series starting with Larklight.
Philip Reeve HATES the term Hungry City Chronicles and calls them the Mortal Engines Books. Or something like that! Check out his website or blog, which seems a lot more up to date.
To get an idea of just how imaginative the books are, click here for a video of some of the cover art.