Primarchs is now available for pre-order, and so is another book I wanted to mention, and thats the Emperor's Gift by Aaron Dembski-Bowden
Gathered within this anthology are four novellas focusing on some of the mightiest warriors and leaders that mankind has ever known – Fulgrim, Lion El’Jonson, Ferrus Manus and the twin primarchs Alpharius and Omegon – and the roles that they may have yet to play in a war which threatens to change the face of the Imperium forever.
The Grey Knights are all that stands between mankind and the ravages of Chaos. Since their secretive beginnings during the Horus Heresy, these legendary Space Marine daemon hunters have journeyed into the dark realms of the warp – and beyond – in pursuit of their supernatural enemies. Through an intensive regime of psychic training, new recruits are brought to the clandestine fortress of Titan to join the hallowed and vaunted ranks of the 666th Chapter. More than ever, these legendary battle-brothers must be vigilant and ever ready to defend the Imperium for the forces of Chaos are never truly defeated, and Armageddon beckons…