Lord of the Night.
Not really a big, Chaos fluff-centric book, but you follow the long lost Zso Sahaal, one of Konrad Kurze's senior commanders and heir to the leadership of the Night Lords Legion on a bloody, terror filled rampage through the dark underbelly of a Hive city, complete with the complex and ugly workings of the business end of the Inquisition and Arbites trying to stop him.
A renegade Psyker Interrogator splits from the Inquisition to join Sahaal, a merciless renegade Tech-Priest plays all sides against one another, you get a reflection on the Night Lords' rise and fall through the eyes of someone who was there to watch it all, the governor of the Hive is murdered by an erstwhile ally, an ancient Eldar plot is revealed and foiled, and an entire world dies in screaming horror as the Night Lords descend to claim an ancient treasure.
...Sounds good, eh?
It's a wonderful look into the darkness that permeates the 40K universe, plenty of great Night Lords material, and overall it's simply an incredible read. Totally worthwhile, whether you use it or not.