Richter Kless
Richter Kless was the in-universe author of the Liber Chaotica, a collection of tomes that sought to explain the madness that is Chaos. Unfortunately, as was the case with all those who took to this task before him, he had succumbed to a curious sort of insanity that led to him seeing things that certainly shouldn't be witnessed by any ordinary man under Sigmar's protection, Inquisitorial Decree or not. Alongside its role of collating and documenting the innumerable forces of the Great Enemy, the Liber Chaotica also serves to document the decline of its unfortunate author's sanity.
Book 1: Liber Khorne
A good chunk of this chapter documents the various tribes of Chaos Vikings, their lifestyles, and their peculiar customs in a detached, informative style. Beyond that is a discussion on how the warriors of the Blood God differentiate themselves from their peers, including their mutations and weapons (one of which includes a goddamn chainsword).
After a disscussion on the similarities between Khorne and Khaine and a description of the Daemons of Khorne, shit gets WEIRD.
Through torturous visions, he sees the Black Crusades of Abaddon as well as the forces of the Berzerkers and the Daemon Engines of the far future-ish. The last segment is an even stranger vision which is a (false) prophecy about the end of the world as we know it.
Book 2: Liber Slaanesh
Book 3: Liber Nurgle
A.K.A. the one where poor Richter starts losing it.
Book 4: Liber Tzeentch
By this point, whatever sanity Richter has been abandoned.
Book 5: Liber Chaotica
This last tome is effectively about Chaos Undivided and the process of how the Everchosen is made.