Attack On Titan: Kuklo Unbound is the second and third of the prequel novels to the AoT manga series. They take place many decades before the regular series begins with the breach and loss of Wall Maria. The English version combines the two novels into one.
The story begins briefly in the midst of the preceding volume, and then jumps about ten to fifteen years (my best estimate) to the time of the "current novels". As the story starts, an infant is found alive in the Titan vomit, having somehow survived the death of his mother. Dubbed "the Titan's son", the boy is sold from master to master and put on display. Beatings, malnourishment, and a hellish existence follow. Until one fateful day, that is...
One day, a wealthy man purchases the boy for his son, Xavi, to beat up to bolster Xavi's confidence so he can prepare to enter the military. Every day is an agonizing beating for the savage Titan's son, named "Kuklo". But one day he meets the kind-hearted sister of Xavi, Sharle.
Sharle takes it upon herself to teach the boy about the outside world and so on. They develop a close bond bordering on childhood love when a series of events force them to flee land eventually force Kuklo to join the re-constituted Survey Corps.
Of course, Kuklo is fine with this, to an extent, as he wants to forever sever the psychological connection that his life has given him to the Titans. With the aid of Cardina and Rosa (who is the daughter of Maria and the late Solm from the previous book) he goes about doing this.
This is like the previous book, and unlike the comics, in that the focus on characterization is much more deep. Don't get me wrong, the manga is big on characterization and not just action, but there is only so much a comic medium can do when it comes to exposing the inner workings of characters. Just as manga is more vivid in the descriptiveness of the world and characters, so prose shows their inner thoughts far better.
I have to say that I loved Cardina. He was snarky and a bit sour at times, but all in good fun. He sort of played the role of Kuklo's lancer and became the closest thing, other than Sharle, that Kuklo has to family or loved ones. The development of their friendship was awesome.
Rosa was cool as as well. Her relationship with Kuklo was kind of like Annie's to Eren's decades later. Well, if Annie were a good guy that is (though we still don't know what the goals of the Titan Shifters' village is, and these are child soldiers in the main manga story, so what their alignment is, we don't know).
Finally, Sharle's friendship, goodness, and developing (I assume) romance with Kuklo was sweet and fun to read. On the subject of romance (or the potential thereof), Angel and Maria from the previous book are living together and Rosa calls him "uncle". Now whether they are just friends or are married or live together or what is happening, I don't know. I like to think they are married. It would be a sweet resolution to their arcs of the first book.
The book was a little more relaxed and laid-back. Not as intense as the manga series in Eren, Mikasa, and Armin's time, so that lent itself to a narrative that was different, but not a whit less fun. In some ways, it was more fun. The adrenaline rush of terror was absent, but the slow terror and dark feel of the setting was heightened.
There is one cool (POTENTIAL) spoiler. Such as references to an outside community that is a myth known only to the nobility.
I have to say that this is possibly my favorite AoT story to date. Well this and the previous one. But they are different enough from the main story that takes place years later in-universe that it's hard to judge such things.
About the only thing I disliked was the inclusion of some of the cheesier (but brief) shonen tropes that not even the original manga had. Other than that, this was a masterpiece of a light novel and one I greatly, greatly enjoyed.