Attack on Titan Universe

Humanity lives inside three concentric walls because the alternative is being eaten. The Titans outside are naked, grinning, mindless and enormous, and for a hundred years the walls held — until the morning something taller than the outer wall put its face over the top of it. What follows is a story about walls: who built them, what they were really for, and what the people inside will consent to become in order to keep standing behind them. The novels here run beside Hajime Isayama's manga rather than retelling it. Before the Fall goes back to the first crude attempts at the vertical manoeuvring gear, and to a boy born inside a Titan's stomach. The Harsh Mistress of the City takes a walled town cut off from the capital and asks what its rulers do once the supply line is severed. Lost Girls gives Mikasa and Annie the days the main story skipped over, and Garrison Girl comes at the wall from a soldier's daughter's angle. The register is siege horror with a political argument underneath, and it does not flinch: the soldiers are children, the orders are lies, and each reveal quietly re-labels who the monsters were. These are companions rather than doors. Read them once the manga has its hooks in, and start with Before the Fall, which is the most self-contained of the set.