Quick review that will act as information to what you can expect.

Basically the story of Levi and how he got to the Survey Crops. It follows him in his underground days with his two companions Isabel and Farlan. His life at this point was him more or less owning the underground in terms of his reputation. Erwin comes along and tries to recruit him. From thereon, you can expect to see the early days of Levi and his team in the Survey Crops.

The story is more or less what Attack on Titan already is. A dark world full of despair. So you can go read it for yourself if you really want to, but I don't think this is canon anyways. This is just one of the earliest views of Levi's experience on the frontlines of humanity, and how the same thing goes over and over again in the main story, that is, the darkness of the AOT world. The same is for art. Multiple scenes of great drawings here and there but I prefer the original manga compared to this. I think there is a slight difference, especially with fight scenes, in that the original is better, but this version has good quality in its own respect.

Quite a nice add on to the story would recommend this if you are a die-hard fan. I believe that there aren't any differences between this and the anime OVA of it. So choose your pick from that.