It really is the time of Warhammer 30k. It seems we're always getting new things for the Horus Heresy, as opposed to furthering the adventures happening in the 41st millennium. Not that there's anything inherently wrong with that, mind you. Just pointing out an observation. If it means cool, new models and books, I'm all for it. Speaking of new books, Forge World is taking orders for The Horus Heresy Book Six, as well as a bunch of new army bundles, over in their webshop.
Much of the fighting during the Horus Heresy was done unseen by most as various Space Marine chapters fought civil wars against the portions of their chapters that broke off. Loyal marines figured if they were to die, if they at least took out one of their former brethren with them, then that was good. If they could take out more, that was exemplary. These Shadow Wars were fought all across the galaxy, and are the focus of Book 6.
The new sets depict these "shattered legions" as groups that split off from one-another and suddenly found themselves fighting an enemy that was very much their equal.