The book acknowledges from the outset that Slaanesh isn't simply about "pleasure". The core emotion can be perverted in to many forms ranging from "purpose" to "willpower" and "experience". These include not only the negative forms such as "lust"and "greed", but can extend to what may seem benign emotions such as "love", "joy", and "aesthetics". Therefore the picture of Slaanesh that is painted is one of the most insidious gods of the chaos pantheon: one who has many ways to infect the hearts of mortals.
The excellent in-depth treatment of the nature of Slaanesh is elucidated through the considerations of his cults. This is the manner in which the Khorne volume should have been presented I think. The book then descends in to a few other matters (a re-writing of the Slaves to Darkness: Brothers von Gottlieb; daemonic possession; the eldar; elves and Slaanesh; and short background writings on various tribes and Warhammer individuals). The strongest section of this has to be the "conversing with the damned" section, wherein Kless interviews a Slaaneshi adherent charged with a variety of (not very well defined, yet obviously implied) crimes.
Together with a presentation and analysis of the daemons of Slaanesh and the Emperor's Children, this book is not only strong, but very well rounded. Superior to the exploration of the nature of Khorne, the book demonstrates in no uncertain terms the full extent of the threat that Slaanesh represents.