Time of Legends
Heldenhammer
Empire
God King (Those are the three Sigmar books)
Warrior Priest
Horus Heresy
1) Horus Rising
2) False Gods
3) Galaxy in Flames
4) The Flight of the Eisenstein
6) Descent of Angels
7) Legion
11) Fallen Angels
13) Nemesis
14) The First Heretic
17) The Outcast Dead
19) Know No Fear
40,000
Eisenhorn Omnibus
Ravenor Omnibus
Pariah
Blood Angels Omnibus
Blood Angels Second Omnibus
Grey Knights Omnibus
Soul Drinkers Omnibus
Blood Ravens: Dawn of War Omnibus
Atlas Infernal
Faith & Fire
The Emperor's Gift
Space Marine Battles Novels
Battle of the Fang
Fall of Damnos
Hunt for Voldorius
As I said recently, I'm reading The Blood Angels - Omnibus, at the recommendation of Argothoth. It's a very fun book. The Blood Angels is one of the Chapters of the Space Marines; the outrageously enhanced soldiers, with their outrageous weapons and armor. It's not overly complex, or deep, but it does try to cover some ground in a few areas. Psy powers; various religious ideas; tech/cyborg stuff...
More important, for me, is the outrageous history touched on throughout the book. Going back nearly 50,000 years (10k before our time), it begins with the origin of God-Emperor/Emperor of Mankind. Here's a tiny bit, and you can read tons more at warhammer40k.wikia.com/wiki/Emperor_of_Mankind if you're interested.
As far as I can tell, most of this is from the game manuals. Possibly some out-of-print books? Does anyone know for sure?The Emperor is the collective reincarnation of all the shamans of Neolithic humanity's various peoples, the first human psykers. The foul Warp entities that would become the four Great Powers of Chaos had not yet fully formed when the Emperor was born on Earth during prehistoric times, somewhere in ancient central Anatolia (modern Turkey) in the 8th Millennium B.C. But even before the birth of the Emperor, as humanity grew and progressed, the Warp began to become increasingly disturbed by the dark undercurrents of humanity's collective psyche, and the shamans began to lose their former ability to reincarnate into new bodies. Instead, upon dying, their souls were being consumed by the entities and daemons of the Warp. Eventually the shamans of humanity, unable to reincarnate, would become extinct, and without the shamans and their psychic abilities to guide the race, humanity would inevitably fall prey to the corruptions of Chaos, just as eventually happened to the Eldar. In these ancient days, all the shamans of Earth gathered in a grand conclave to decide what must be done to stave off the day when they had all been consumed by the Warp.
In the end, the shamans decided to pool their collective psychic energies by reincarnating as a single soul in a single human body to create an individual they called "the New Man." The thousands of shamans, as one, took poison, and as one, they died, their souls flowing into the Immaterium in a rush of psychic power that overwhelmed those daemons who sought to feast upon it with a cleansing, purifying fire, a flame imperishable that became one soul out of many. A year later the child who would become the Emperor was born in a Neolithic settlement of Anatolian herders and farmers. His psychic power was so great that its energies altered his genome and physiology in the womb and rendered him immortal so he would no longer need to reincarnate and could not be assaulted by the daemonic creatures of the Immaterium upon his death. As he grew older, his potent psychic powers began to manifest. Over the many millennia of his life, he travelled among the different peoples of Mankind, using his ancient wisdom to help where he could in the guise of many different benevolent persons from human myth, religion and history. But as his psychic powers further developed, he became ever more aware of the terrible dangers that awaited Mankind in the broader universe and he resolved to do all in his power to defend and guide humanity towards a future as the predominant species in the galaxy. As more and more humans were born with the mutant psyker genes that granted them the ability to wield the potent power of the Immaterium in the last centuries of the Dark Age of Technology, and humanity suffered from the deadly effects of uncontrolled psykers, the Emperor realised that he would have to take a more direct and open role in human affairs than ever before.
However, in the current Warhammer 40,000 background story, the Emperor's origin and history prior to unifying Terra is left largely mysterious and undetailed, though his immortality and extraordinary psychic abilities remain intact. The first mention of the Emperor in Imperial records is when he unified Terra at the end of the Age of Strife in the late 29th Millennium. Horus mentions that the Emperor lived "in Anatolia, in his own childhood" when talking of his first meeting with the Emperor. It is known that he had been immortal and ancient even before his ascension to the Golden Throne over 10,000 Terran years ago. The Emperor is the "New Man", the first and greatest of the new race of human psykers. He is also the collective reincarnation of the extinct shamans, sorcerers and wisemen who had guided primitive humanity during prehistoric times. As the Emperor grew older his powers began to manifest themselves and become more potent and he gradually remembered his thousands of past lives adding all of their knowledge and experience to his own.
For thousands of years before becoming the Emperor, he guided and watched humanity develop over the course of its history, assuming the guise of a large number of historical personages. He was aware that the darker extremes of human nature were feeding the growth of the Chaos Gods in the Warp, and so he sought to promote peace and harmony on Earth and thereby curb the growth of the Ruinous Powers' strength. The Chaos Gods themselves recognized the Emperor as their greatest enemy among all the intelligent beings of the galaxy. Only at the end of the Age of Strife did the Emperor emerge from obscurity to take a more direct hand in the future of humanity, conquering the warring factions of Mankind's homeworld and establishing his direct rule over the Earth. The Emperor accepted the deaths of the many innocents that resulted from his conquest with great remorse in order to achieve the greater good of unifying humanity and protecting it from the manifest predations of the Warp.
With the assistance of the Adeptus Mechanicus on Mars, who joined with the Emperor and the people of Terra in the Treaty of Mars that formally founded the Imperium of Man in the 30th Millennium, the Emperor created the first Space Marines and fleets of interstellar starships that would carry his armies across galactic space. The objective was a Great Crusade that would unify all of the planets colonized by Man during the Dark Age of Technology prior to the Age of Strife into one Imperium of Man, and also subdue, destroy, or force into exile all intelligent alien races from the Milky Way Galaxy, what was to become the Imperial Domain, the manifest destiny of Mankind. The Emperor also created the superhuman Primarchs from whom the Space Marines' gene-seed was later developed to serve as his primary military commanders for the Great Crusade. The Chaos Gods, however, sought to thwart the Emperor's grand plan. The Primarchs were sucked into the Warp even as they gestated in the gene-laboratories deep beneath the Imperial Palace, and were scattered across the inhabited worlds of the galaxy. During the Great Crusade all but two of the twenty Primarchs were found and united with the Space Marine Legions that had been created after their disappearances from the genetic material that they had left behind. As the Emperor traveled across the stars, some humans wanted to worship him as a god, however he forbade this, proclaiming "I am not a god; rather than enslaving humanity I want to free it from ignorance and superstition." However, Lorgar, the Primarch of the Word Bearers Legion, desperate to find some outlet for his belief that Man must have a God to worship to be truly whole, gave in to the constant whispers of the Chaos Gods and, after corrupting his Legion to their service, sent his First Chaplain Erebus to poison the minds of the other Primarchs as well as their Legions. Just as the Imperium had reached its apex in the early 31st Milennium, the Emperor's most trusted son, the Primarch Horus of the Luna Wolves Legion (later renamed the Sons of Horus), fell to Chaos as a result of his own pride and ambition and betrayed the Emperor, and along with fully half the Space Marine Legions, initiated a massive civil war for control of the galaxy. This rebellion is known to history as the Horus Heresy. Though the Emperor ultimately defeated Horus during the Traitor Legions' assault on Terra, he was all but slain in the battle suffering from a crippling loss of limbs and mortal systemic damage; only the life-supporting Golden Throne has sustained his living corpse in a kind of stasis, neither dead nor truly alive. Trapped within his prison of flesh, only the Emperor's mind is allowed to wander free within the Immaterium, still seeking to protect and guide humanity to an increasingly distant better future.
And what I'm wondering is if anyone has particular recommendations. I'm going to try the Horus Heresy series. I bought the second book for a song at the Borders closing, and have the first book on order at B&N. I also have the first books of the Nagash (I guess that's not 40K) and Sigmar (maybe also not 40K? But it may have a lot of his origin in it?) on order. Anybody else have favorites? There's, like, a thousand books, and I'm sure many are not necessarily great.