I've just finished Aaron Dembski-Bowden's 40K novel exploring the beginnings of the Black Legion, and the first in the series of the same name, entitled The Talon of Horus.
The book is set some indeterminate number of centuries after, but within a millennium of, the Horus Heresy. By this time the traitor legions defeated at Terra have all retreated into the Eye of Terror, and the Imperium has mostly forgotten about them and become accustomed to living under their undead god-emperor, now entombed on the Golden Throne.
It is told via the point of view of a onetime legionnaire of the Thousand Sons, Iskander Khayon, and has him laying out the story as a framing device to the Holy Inquisition on Terra, to whom he has willingly surrendured himself. Khayon begins the account with a short explanation of the state of the Nine Legions in the Eye, the ongoing Legion Wars, which pit them all against one another, for old grievances or for the glory of their respective Chaos gods, in some cases. We're introduced to Khayon's retinue, and before long he forms a loose band of other traitor space marines to go on a hunt for a weapon to foil the Emperor's Children and their plan to dominate the rest of the Nine Legions by cloning Horus, whose body has been kept by his former legion, until only recently having been taken in a raid by the Emperor's Children.
Over the course of the story, Khayon and crew come to meet Ezekyle Abbadon, former first captain of the Sons of Horus and right hand of the man himself, who goes on to be the main frontman of Chaos Space Marines in the 41st millennium, and the rest is fake history.
It was a pretty entertaining book for one somewhat versed in the overall lore of the setting, but I don't think I would recommend it as a place to begin for neophytes to 40K. Also, the Abbadon we see here is a wholly different person than the one present in the Horus Heresy series. Fair enough, it's eons later, and the man has been through some momentous things in the intervening years, but the only real explanation we have other than the obvious fact that time passes and things evolve and change is that at some point he went on a pilgrimage of worlds in the Eye, and through overcoming various trials basically reforged his personality, taking a lot of the edge off, while retaining and even bolstering his natural charisma and leadership abilities. He is able to compel loyalty and subordination here in a way not unlike how Horus and the other primarchs are described to so in their day.
Next up on my agenda is the next book in the series, simply called Black Legion.