David Annandale
Roboute Guilliman: Lord of Ultramar
Primarchs (series) part 1

Roboute Guilliman, the Battle King of Macragge, leads his Legion, the mighty Ultramarines, in conquest of the galaxy as part of his father's Great Crusade. Faced with an alien empire, all his dreams of a shining galaxy of peace threaten to fall into ruin...
The Horus Heresy
novel

Also in:
Primarchs: the Ebooks Volume 1
Nine Loyal Primarchs
Primarchs: the Audiobooks Volume 1