Novels set years before the beginning of the manga.

1) Anheru works at a workshop creating protective gear/weapons for the scouting legion (the story is set before the 3D maneuver gear was invented). Anheru is a first class weapons craftsman, and therefore has direct access and also is involved in the ongoings within the scouting legion. In the story, people still didn't know about the weak points of the titans; the story is about Anheru beginning to work on the ultimate weapon/gear to fight the titans. So essentially we learn about how the 3D maneuver gear came about.

2) A baby is found crying in a mass of Titan vomit, amidst the dead titan flames. The child is called "child of titan" and is essentially hated by the people inside the walls. He also seems to grow unnaturally fast. Named Kyukuro, he parts himself from his past/upbringing and gambles on the fate of humanity by enlisting in the scouting legion.

3) Kyukuro has survived from the outside wall: in order to defeat the titans, he gets his hands on the 3D manuever gear. Although the gear is still imperfect and causes a troublesome incident for Kyukuro, he fights the titans. Whether it is because it is his fate to fight them, or it is because of his past, he fights against them.

(Source: Mangahelpers)