Well folks it looks like Amazon has the Title & Synopsis up for Guy Haley's entry to the Siege of Terra! I had no idea where to drop this so figured let's keep things tidy and start a new thread specifically for this book...
The Horus Heresy: Siege of Terra: The Lost and the Damned
On the thirteenth day of Secundus, the bombardment of Terra began... With the solar defences overcome through the devastating strength of the Traitor armada and the power of the warp, Horus launches his assault on the Throneworld in earnest. After withstanding a ferocious barrage of ordnance, an immense ground war commences outside the Palace with every inch gained paid for in the lives of billions. The front lines are beyond horrific and the very air is reduced to poison and blood. Bodies are thrown into the meatgrinder but the outer redoubts cannot possibly hold for long, even with the loyal primarchs to reinforce them. For Horus has his own generals to call upon... Between the plague weapons of Mortarion and the fury of Angron, the defenders face a losing battle.
Can we tentatively assume that 2 books a year is the Siege schedule putting the end somewhere in the region of 2022?