Book 17 of "The Horus Heresy: Primarchs". Fantastic character focussed novel featuring the Primarch of the Blood Angels legion.
Sanguinius is the Great Angel, most beloved of all the primarchs, his mighty exploits celebrated throughout the entire Imperium as the Crusade expands into the void. And yet the origins of his Legion are shrouded in mystery and rumour, his unique physical form is an enigma, and his perilous home world remains off-limits to all but his own secretive people. When a discredited remembrancer arrives with the expeditionary fleets to chronicle the primarch's deeds, he has to work hard to uncover the truth behind the legends. As he accompanies the Ninth Legion to war against the enemies of the Emperor, he eventually learns much more than he expected to, not just about the subjects of his study, but also the nature of the Imperium itself.