A couple of years ago I spent a very pleasant few days with Jeremy Levett in Bristol, coming up with something called The Traction Codex. It's a sort of encyclopaedia/history of the World of Mortal Engines, featuring all those things you Always Wanted To Know But Could Never Be Bothered To Ask, like, how did Airhaven get airborne? Why do the cities use heavier-than-air fighters while the Green Storm stick to airships? Who was Red Loki? etc, etc. We've also added some details which never made it into the books, like the alarming sport of 'Traktionturnieren' or civic jousting...
If you've bought an e-book edition of the Mortal Engines books recently you may have found The Traction Codex attached to it as a sort of appendix, but if you already own the books and still want to look at the Codex, your luck is IN: it's now available separately on Amazon (UK). It even features a few illustrations by yours truly, like the turnierenstadts above, or this one of Batmunkh Gompa:
Because of the way it's been bundled up with the books, it proved quite tricky to write - we had to leave a lot of details because we wanted to avoid spoilers for people who might be referring to the Codex while reading Mortal Engines, and wouldn't want to learn what happened to characters in the later books. So it's by no means a complete guide, but I'm pleased with the way it turned out; Jeremy was able to provide some rather elegant escape-routes from the logistical corners I'd painted myself into while writing the books, and it was nice to have a chance to expand the Mortal Engines world a bit without writing a whole 'nother novel - as much as I'd still like to do that too... I'd also like to come back to the Codex in a year or so, add a lot more entries, and publish a paper version with extra illustrations, but whether I'll ever have the time and energy we can only guess.
In the meantime, The Traction Codex is a snip at only 85p, and it's available HERE.
EDIT: I didn't realise that Kindle books have 'regions' like DVDs. This version of The Traction Codex won't be readable if you're in the USA (and if you're anywhere outside the UK it's probably best to check that it will be compatible before you splurge 85p on it). Hopefully Scholastic will one day make a US version available too.
(NB: As with the current UK editions of the Mortal Engines books, the cover is pretty repellant: I had no control over that, and can only apologise. It also very shabbily fails to credit my indispensable co-author, Jeremy Levett.)