The Bone Trilogy consists of three parts: Bone, Scarred and Unbroken. The first story introduces Dean as the one tasks to torture an angel captive in a post-apocalyptic world destroyed by the angels themselves. It gets gritty, gory, and pretty graphic.
And let us not forget the Stockholm Syndrome, people! Oh so delicious how our angel falls - both in power and sanity.
The relationship is kind of disturbing, but not too much (and it makes sense in a way only evident in Supernatural).
Anyways, Bone is told from Dean's perspective and focuses on the mental breakdown of Dean's captive. Scarred is told from Castiel's perspective, and we see the development of something...more, between these two. Unbroken is told from the perspective of several key characters in the Supernatural world, where we see a third-person perspective on twisted relationship between Dean and Castiel. And it's implication in this destroyed world.
The first two parts focus on the relationship, while Unbroken is more plot-centric. I enjoyed the characterization of all the characters in how they fit the mold of their own circumstances. The story diverges from canon, becoming an AU that hints at things that are canon-compliant. Therefore, it would be best that readers have at least watched Supernatural up to season around season 6.
Anyways, The Bone Trilogy is an example of excellent fanfiction: pretty well proofread (some mistakes, but few), good characterization, and well implemented elements (i.e. Stockholm syndrome!).