The Emperor's Legion (Watchers of the Throne #1) by Chris Wraight 16, May

Cadia has fallen, the Great Rift has cut the Imperium in half and Holy Terra faces a threat not seen since the days of Heresy. 

Set between the Gathering Storm that brought the overarching Warhammer plot forward for the first time in over a decade and the beginning of the Indomitus Crusade that has provided the groundwork for the modern setting, Chris Wraight's The Emperor's Legion focuses on events on Terra during this tumultuous time.