Attack on Titan: Before the Fall

Adaption of Ryō Suzukaze's light novel series of the same name.

The story takes place before Hajime Isayama's original manga and follows Anheru, a member of the Survey Corps who works in weapons development. Anheru is tired of seeing the corpses of his fellow Survey Corps members, and is working to design equipment that will allow humans to be able to kill Titans. Thus far, no one has been able to find out the weak point of the Titans, and so no one has managed to kill one. However, government conservatives think that fighting against Titans is unnecessary, and order the dissolution of the Survey Corps.

(Source: ANN)

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)

Attack on Titan

A century ago, the grotesque giants known as Titans appeared and consumed all but a few thousand humans. The survivors took refuge behind giant walls. Today, the threat of the Titans is a distant memory, and a boy named Eren yearns to explore the world beyond Wall Maria. But what began as a childish dream will become an all-too-real nightmare when the Titans return and humanity is once again on the brink of extinction … Attack on Titan is the award-winning and New York Times-bestselling series that is the manga hit of the decade! Spawning the monster hit anime TV series of the same name, Attack on Titan has become a pop culture sensation. 

(Source: Kodansha Comics)

Volume 3 contains the special story "Rivai Heishichou" (リヴァイ兵士長, Captain Levi).
Volume 5 contains the side story "Ilse no Techou" (イルゼの手帳, Ilse's Notebook).

Attack on Titan: Before the Fall Prologue

A 12 page pilot chapter to the side-story Before the Fall. Some events differ from the original side-story.

Attack on Titan: Before the Fall Prologue Franchise