Novel is set years before the beginning of the manga.
Angel works at a workshop creating protective gear/weapons for the scouting legion (the story is set before the 3D maneuver gear was invented). Angel 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 Angel beginning to work on the ultimate weapon/gear to fight the titans. So essentially we learn about how the 3D maneuver gear came about.
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 Kuklo, he parts himself from his past/upbringing and gambles on the fate of humanity by enlisting in the scouting legion.
Shingeki no Kyojin: Before the Fall was published in English as Attack on Titan: Before the Fall by Vertical, Inc. from September 16, 2014 to May 26, 2015 with the latter two volumes released as a 2-in-1 omnibus titled Attack on Titan: Kuklo Unbound.