The prequel novels to Attack on Titan—and the basis for the Attack on Titan: Before the Fall manga series!
In Volume 1, Before the Fall, we learn the origins of the devices that humanity developed to take on the mysterious Titans. Volume 2, Kuklo Unbound, details the trials of “the Titan’s son,” Kuklo, and his journey into the Survey Corps and to master the equipment that could be humanity’s best hope!