Plot details for all the books bar Eye of Terra have been released;
Originally Posted by Book 36: Path of Heaven
For too long had the Vth Legion ranged out beyond the sight of the wider Imperium, remaining ignorant of the Warmaster’s rebellion and the war that inevitably followed. Only once their primarch, Jaghatai Khan, had satisfied himself that the path before them was just and true did the White Scars choose a side, taking the fight to the traitors on every front. But, four years later, the Legion’s unfettered spirit has been broken by relentless attritional warfare against the Death Guard and the Emperor’s Children – the Khan’s Stormseers must find a clear route to Terra if they are to take part in the final, apocalyptic battle.
Originally Posted by Book 37: The Silent War
While loyalist and traitor forces clash on a thousand battlefields across the galaxy, a very different kind of war is being fought in the shadows. Rogal Dorn and his Legion prepare to defend the Solar System against the armies of Warmaster Horus, even as Malcador the Sigillite charges his many agents and spies with missions of the utmost secrecy. The future of the Imperium is being shaped by unseen hands... This Horus Heresy anthology contains fourteen stories by some of Black Library's top authors, including James Swallow, Chris Wraight, Anthony Reynolds and many more.
Originally Posted by Book 38: Angels of Caliban
Two infamous Space Marine Primarch rivals clash for the final time.
With the Dark Angels spread across a hundred systems, primarch Lion El’Jonson stands as Lord Protector of Ultramar – though his true motives are known to few indeed, and old rivalries on the home world threaten to tear the Legion in half. But when word comes of the Night Lords’ attack on Sotha, the Lion’s brutal actions bring Imperium Secundus once again to the brink of civil war. Not even the most fearsome warriors of the Dreadwing, nor any arcane secret of the Order, can guarantee victory if he sets himself against his loyal brothers.
Can't 100% confirm but sources say that after Angels of Caliban the next novel, number 39 in the series, will be Praetorian of Dorn by John French, due for a 2016 hardback release and a January 2017 paperback release, and is about the Imperial Fists fighting the Alpha Legion in the Ghost War for the Sol System.