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.
The Liber Chaotica (as we have it) is a compilation of these notes by the order of an Inquisitor who sought to understand Richter's slippery slide.
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 Khorne Berzerkers and the Daemon Engines of the Dark Millennium. The last segment documents an even stranger vision which is a (false) prophecy about the end of the world as we know it.
Book 2: Liber Slaanesh
The first part of this book take to describing the nature of the Lord of Excess, in a way that's beyond just the meme-ridden "Drugs & Sex" business, and how one can even describe excess. Following that is a discussion on the Cults of Chaos: how they operate, who leads them, and how they manage to infiltrate civilized territories, and how people get convinced to follow them.
The next part discusses a tale of two brothers, one who fell to Slaanesh and turned into...something less human, and one who joined Khorne and became a Daemon Prince. Following that is an in-depth description of an exorcism and the many rituals that take place during this occasion.
The most intriguing segment involves Richter's conversation with a debauched Bretonnian noble who formed his own pleasure cult before getting busted by the Witch Hunters and sentenced to death. This noble seems to hold no resentment or remorse for what happened, and doesn't so much as hate the Cult of Sigmar as he does find it completely boring. Instead, he tries to humanize Slaanesh and preach that the Chaos Gods aren't entirely evil. Indeed, he was so interested in Richter that he gives some poetry in hopes of convincing the author about his views.
Following that is a dive into the relationship between the Elves and the dark gods, how the Dark Elves are totally not Chaos, and in particular how Aenarion, first king of the High Elves, became a vessel for Asuryan. After this is a description of those tribes of warriors enslaved by Slaanesh, with particular segments dedicated to Styrkaar, Dechala the Denied One, and Azazel. His discussion on the daemons of Slaanesh are written as he finds himself again stuck in the Realms of Chaos, witnessing their pleasures.
His visions of the Dark Millennium see him describe the Noise Marines as well as the Birth of Slaanesh, as perpetrated by the Eldar.
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.