My thoughts on Attack on Titan: Before the Fall, by Ryo Suzukaze. This is a light novel which serves as a prequel to the AoT anime and manga.
From the Back Cover:
Before the fall, and before the trials of “the Titan’s son” Kyklo, a young smith by the name of Angel Aaltonen grappled with the giants as only a craftsman could…
The first of the three-part Before the Fall light novels, whose second and third installments have been adapted into a manga of the same name, this prequel of prequels details the origins of the devices that humanity developed to take on the mysterious Titans.