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).
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.