Two points are worth making right out of the gate in re this two-part light novel set in the Attack on Titan universe, brought to us in English courtesy of Vertical, Inc. First is how much better it is in most every respect than the previous Attack on Titan light novel, Before the Fall, far better in the concept than the execution. Second is how it provides the best evidence yet that Attack on Titan is becoming, with startling swiftness, a open-world franchise on the order of Star Trek. In fact, arguably even more open-ended, since Star Trek's expanded-universe productions like its spin-off novels all feature at least one major, canon character at their center. Before the Fall and Harsh Mistress use the Titan setting, but not a single canon character from the original series appear in them; they're arenas in which new characters are deployed to confront (if not always successfully) variations on the themes that bubble through that universe.
The bare outlines of the story are simple enough; the implications brought up by the story less so. Childhood friends Rita Iglehaut and Mathias Kramer have vastly dissimilar futures waiting for them: she's an aspirant for the Garrison that defends humanity against the encroachments of the monstrous Titans, he's the son of a merchant who has his child's future mapped out for him in exact detail. When Rita goes missing after the township she's stationed in, Quinta, is cut off from the rest of humanity and surrounded by Titan encroachment, Mathias becomes hellbent on enlisting and going out there to help her. But Mathias's father has exercised a little social engineering to prevent the boy from joining the ranks of the guard, and so the boy has to cheat.
Cheat he does. His father's mansion back in Quinta was stuffed with all manner of antiques and objects d'art. If he can find someone willing to take him to Quinta, they'll get looting rights to the collection. It's a short-sighted and desperate plan, but Mathias is young and impulsive, and so buddying up with a corrupt former guardsman and his criminal cronies actually seems like a good idea. Too bad Mathias has no idea what such a partnership will cost him — his own innocence, mainly, which vanishes along with the life of a guard whom he kills in a foolish panic.
What Mathias doesn't know is that Rita has lost a great deal as well. After Quinta lost contact with the outside world, she was saddled with the responsibility of keeping the town from disintegrating into anarchy — sometimes via measures as blunt as wading into crowds of looters and cracking their heads together. By the time Mathias shows up, she's no longer the woman Mathias remembers; she's a homebrew dictator who sees feeding criminals to a capture Titan as a sensible way to enforce order. It breaks Mathias's heart to join an underground resistance against her, but he can't see any other way to stop the suffering she's caused. He is also blissfully unaware of how Rita may well have anticipated how any resistance could arise.
Mathias and Rita at first are set up as studies in contrast: she being the one willing to do whatever is needed to keep things together, he being the naif out of his depths. By the time he finally achieves what he's come all this way for, it hardly seems worth it. Too bad the book's climax and denouement is unwilling to go all the way home with such a conclusion; it deploys a deus ex machina, even if it is one that fits with the overall logic of the setting and the situation. But worse, it follows that up with the kind of maybe-they-can-be-happy-after-all ending that completely ducks out on the implications of everything it had taken such time and pain to set up. A more honest ending would have underscored all the more how their paths had diverged permanently. Seems even as purportedly hard-nosed a property as Attack on Titan can't avoid being moon-eyed when the moment demands it.
Titan has been described elsewhere in terms like "Japan's answer to The Walking Dead". The obvious reason why is because both franchises are about stubborn little pockets of humanity asserting their right to exist in the face of a humaniform (but not human) menace. Less obvious, though, are its critiques of the social structures that spring up to protect humanity in the face of existential crisis. Such institutions are typically tyrannical, and they share an attribute with their real-world counterparts: their reach and lifespan can be extended perpetually in the defense of whatever principles can be held in the highest regard — security, liberty, freedom, what have you. (There has never been a time when humanity was not in crisis of some kind; the idea that until the situation improves we need to give up a few little things, like freedom of criticism or movement, is the mythology of fear-mongering.)
I haven't yet seen the live-action Titan films, but my understanding is that they make explicit the authoritarian — read: fascist — nature of the government in its world. Rita and her iron fist are an incarnation of it under circumstances that are extreme even by the rules of Titan's worldbuilding, but I suspect we'll be waiting a while for the last word on what the franchise's actual point of view is on this tension. If in the end resistances like Mathias's are regarded with a "Yes, but ... " and a slew of nattering about how any social order, even a bad one, is better than none, consider me let down. The last thing we need is a major-league entertainment franchise that amounts to an apologia for despotism.