On the daemon world of Torvendis, deep in the heart of the warp storm known as the Malestrom, ancient rivalries threaten to shatter the delicate balance of power, currently held by the Lady Charybdia, daemon princess of Slaanesh. When the warriors of the Word Bearers arrive on the planet, hunting one of their own, the traitor Karnulon, monstrous forces are unleashed that could tear Torvendis apart.
Read it because
It's a Black Library classic (originally published in 2003, in fact) and provides an unparalleled insight into the madness of the daemon worlds of the Eye of Terror - as you'd expect from master of Chaos Ben Counter.
This is one of my favorite books from the 40k universe. This book grants us a glimpse of how at least some of the mortals under the sway of the ruinous powers live on a daemon planet. Ignoring a few plot holes the story is overall good and worth a read.