10 Mar 2018
In 2015-2016 I spent a bit of time trying to do some reverse engineering on Dark Souls files.
There was not much activity back then beside Rivernyxx PoC of a mod tool that could render maps, and a few of his 010 templates available online. To try to replicate the dynamics of the community-managed WoW dev wiki, I set up the Dark Souls Dev wiki in an attempt to document file structures, and as a beacon in the dark to find people interested in helping me in the process, without much success. As a Mediawiki instance is quite the pain to maintain, I shut it down recently, but I kept a read-only version available here: DarkSoulsDev wiki archive.
Edit from 2020: boy oh boy, how the time passed. Since around 2018 there has been a lot of activity around Dark Souls modding, and From Software games in general. A massive reversing effort has been done on almost every known file format used by FS and a lot of tools have been created. I'll just leave you a link to the Github wiki Soulsmodding Wikidot that has been collectively maintained by a lot of great folks.