After thousands of years of expansion and conquest, the human Imperium is at its height. His dream for humanity accomplished, the Emperor hands over the reins of power to his Warmaster, Horus, and heads back to Terra. But is Horus strong enough to control his fellow commanders and continue the Emperor’s grand design, or will such incredible power corrupt him?
Read it because
It all begins here. Witness Horus the hero, humanity’s greatest champion and the Emperor’s favoured son, before it all goes horribly, tragically wrong…
The eBook edition includes integrated illustrations of characters and events from the story, and an afterword that places the story in context within the series.
YES 54! the numbers of novels in this serie, telling the story of the betrayal of the emperor by Horus 10 000 year before the current warhammer 40K setting.
For a less daunting introduction to the grim far future universe you can try (starting from the books that dont require prior knowledge of the 40k setting to enjoy):
Eisenhorm trilogy follow the inquisitor Eisenhorm on his hunt for heretics and demons, doesnt require knowledge of the universe
The space wolf omnibus best for an introductions to the world through the eyes of the infamous space marines. Follow the recruitement/creation of those human war machines. no prior knowledge of warhammer needed
Gunt’s ghost follow Comissar Gunt as he lead an imperial soldier company. Good for giving the perspective of more ordinary soldiers on the 40K universe
Also : Ciaphas Cain : unusually comedic for the 40k universe follow Cain a cowardly commissar who’s actions keeps getting him ever more dangerous duty assignments
After that :
Ravenor the omnibus : follow Inquisitor Ravenor and his followers as they investigate a daemonic conspiracy that stretches across space and time
The Uriel Ventris chronicles : Uriel Ventris takes command of the Ultramarines Fourth Company, more space marines goodness also a warhammer 40K beginner friendly serie
Cadian Blood more imperial soldiers grunts pitted against the unspeakable horrors of a Nurgles plague
Bloodlines : a kind of hard-boiled detective thriller in a hive city
Warhammer Horror serie : 30 books collecting horror novels/novellas set in the Warhammer 40k universe
Necromunda : a serie of unrelated books by various author also set in a hive city
Requiem infernal for a single novel following a chapter of the Adepta Sororitas a female only division of the Ecclesiarchy the state church of the imperium of man
Nights lords trilogy : for a view from one of the traitor legion having betrayed the emperor during the horus heresy to worship the chaos gods
Word Bearers omnibus : another serie centred on a chaos space marines chapter
The siege of terra serie : for the final stage of the horus heresy storyline (7 books)
The beasts arise set in warhammer 30k like the horus heresy but 2 millenium later ; the orks forces invade the imperium 12 novels collected in 3 omnibus
The Talon of Horus : two books set in the year 41999, the rise of Abaddon, successor to Horus and Warmaster of the Black Legion.
The Infinite and The Divine for a single novel spanning 10 000 years from the viewpoint of the necrons the oldest humanoid race of the galaxy turned into undead robotic skeletal long in the past
There is more on the site if you search the 40K universe tag