The Unremembered Empire by Dan Abnett

Believing Terra has already fallen to the Warmaster, Primarch Roboute Guilliman founds the Imperium Secundus. The unthinkable has happened - Terra has fallen to the traitor forces of Warmaster Horus Nothing else could explain the sudden disappearance of the Astronomican's guiding light at the heart of the Imperium, or so Roboute Guilliman would believe. Ever the pragmatist, he has drawn all his forces to Ultramar and begun construction of the new empire known as Imperium Secundus. Even with many of his primarch brothers at his side, he still faces war from without and intrigue from within - with the best of intentions, were the full truth to be known it would likely damn them all as traitors for all eternity.