The Traction Codex is an e-book published by Philip Reeve, originally available only if the entire Mortal Engines Quartet was bought at once via the Kindle or KindlePC.
Troction cudecks

The improvised cover

It contains interesting official and new information, such as a glossary listing a variety of cities not properly explored before, a detailed timeline and an introduction.

The book was later made available as a stand-alone e-book, and Philip Reeve has expressed his wishes to publish it in the form of a real book, with additional illustrations and information.

Mortal Engines Series
Fever CrumbA Web of AirScrivener's Moon
Traction City
Mortal EnginesPredator's GoldInfernal DevicesA Darkling Plain
The Traction CodexThe Haunted Sky
Mortal Engines (film)Mortal Engines (Game)