The Traction Codex is a fictional reference book set in the world of Mortal Engines, written by Philip Reeve and Jeremy Levett. Described as 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.
Philip Reeve has expressed his wishes to publish it in the form of a paper book, with additional illustrations and information. This wish will finally be realised in the forthcoming Illustrated World of Mortal Engines.