When I started reading Warhammer 40,000 novels, it was nearly without any context for what I was getting into. I knew I wanted something sci fi, but also something I could play. I'd grown weary of the constant disappointment from Star Trek and Star Wars. Dune is pretty reliable and has gaming options, although I admit that its reliance on House Atreides starts to wear thin after a while.
Warhammer looked like it had the right atmosphere to suit my tastes, and I'd seen novels at my local library so I knew it had an extensive book line. I got started with The Horus Heresy, which was a great place to start for background on the Space Marines but an awkward place to start for getting a sense of the 41st Millenium (because it doesn't take place in the 41st Millenium, and the Empire is a very different entity then).
I think Warhammer 40K is an interesting world. It's definitely not for everyone. If you're thinking about looking into it, here are three things I think you need to know.
Broadly, Warhammer stories are often about humans and aliens (collectively called "xenos") fighting. Sometimes, it's about humans fighting other humans, or xenos fighting xenos, or chaos demons fighting pretty much everyone.
There are three divisions of humans:
Within the Imperium and Chaos, there are countless factions of human soldiers and organisations. There are millions of populated worlds in Warhammer, so the depth of culture and diversity is intentionally inestimable.
The Astartes (ah STAR teez) are transhuman. They're genetically engineered superhumans built and bred for war. They're the guys in the enormous suits of power armour, and they're kind of Warhammer's most ubiquitous character type. Being ubiquitous doesn't make them the "good guys", because there are no good guys in Warhammer (more on that later).
Rogue traders don't seem to come up much any more, at least in what I've read. You can play one in the Warhammer: Wrath & Glory RPG, if you're into roleplaying games. Rogue traders aren't important, but in that way they're representative of what actually powers the Imperium and Chaos and a bunch of different alien races that dominate many of the stories. There are millions of billions of normal humans, or humans who are mostly normal but for a few government-issued cybernetic implants (augments), out there in the Warhammer universe.
They're not all rogue traders. Most, in fact are just hab-dwellers, living and working on a planet dedicated to producing some vital resource for the Imperium. They, like you and I, are the peasants of the setting. They're the cogs in the machine making the mundane things work.
Warhammer 40K isn't hard sci fi. The Emperor is a psychic, and has basically ascended to godhood. There are demonic beings crossing into the universe from the Warp. You can try to focus on the sci fi parts, but the fantasy stuff breaks through more often than not. Magic exists in the Warhammer setting, and it's not the kind that you can dismiss as something so advanced that it seems like magic. It's literal magic, it breaks the laws of physics, it's not technology in disguise. while some stories emphasize that more than others, it's well and firmlyl established.
There's no AI in Warhammer. Well, there may be somewhere I guess, but mostly AI is seen as dangerous. Furthermore, a cult called the Mechanicus controls all technology, and they worship an entity they call the Omnissiah. They see AI as an affront to their god, so they refuse to allow it. (Why this machine cult exists without resistance from the one true god, the Emperor of Man, is a little mysterious. A cynic might say that it's convenient for the Emperor to be lenient to the Mechanicus because he needs an IT department.)
You've probably seen a nature documentary before. First there's a shot of a beatiful lion relaxing in the field, and that makes you happy. Then David Attenbourrough's voice explains that their population is dwindling because of poachers, and you feel really bad. And then the lion gets up and pounces on a beautiful gazelle prancing by and rips it to shreds, and suddenly you feel bad again, but this time not out of pity for the lion but out of hatred.
That's Warhammer, except it's aliens against humans against other humans against demons against other aliens.
It's legitimately morally challenging. You feel good about a book's protagonist right up to the moment you realise that the only reason you want the protagonist to succeed is because it happens to be the protagonist of this particular book. It's very likely that the book you read next will feature that same faction as the main antagonist. There is absolutely no purity in the Warhammer 40K universe. If you're looking for a hero or a messiah figure, then you're in the wrong setting.
Nearly everyone in Warhammer is a mortally xenophobic religious zealot, and totally focused on war. Their days are spent in service to, essentially, a lich, or else to a dark god from outside the universe, or some bestial desire to consume somebody else's life. There are few redeeming qualities, from the perspective of a 21st century reader sitting in a cozy reading chair.
Why would anyone read this stuff? Well, there's a lot of subtext in Warhammer. First of all, it's satire in the same way Judge Dread and Robocop are satire. It's absurd ultra-violence and gore and horror and militarism set in a malicious world, in hopes of demonstrating how bad fanatacism really is. Secondly, it exists primarily as a game, but both games and drama love conflict as a central theme.
Whenever I think about exploring a new comic book series I, more often than not, get overwhelmed just by trying to figure out how to get started. (For the record, the key is to just start, but really comics were just meant as an example.)
Warhammer can make you feel the same way, but believe it or not that's a good thing. I assume that if you've heard of Warhammer, then you can probably figure out how to explore it further. However, I can at least save you from having to do a bunch of rudimentary research. Warhammer is kind of a world within itself, with lots of different geeky hoobies as entry points.
I don't think Warhammer is for everyone. It's a very specific kind of fiction, the games are fairly complex, and the miniatures take a lot of time and effort. If you find that you're intrigued by the lore, though, then you might it satisfying. One of the reasons I've come to appreciate Warhammer is that it's a surprisingly big property. You can buy a book every two months and enjoy Warhammer that way for a reasonable price. You can buy a video game once a year, and you get as much play time with that as you want. You can buy a boxed game and play that for years. Or, if you're rich and bored, I guess you could just buy everything.
You have options for how you commit, and that's nice.
But if Warhammer has too many skulls and hulking marines and icky aliens for your tastes, then you can ignore it entirely and try a different game. Wargaming doesn't have to be about war. It'll definitely be about conflict, but there are wargame-style dungeon crawl games as well as exploration and adventure games. You might be surprised at what's out there.
And if you land somewhere in between, you can of course fashion a mix of your favourite elements of each. I never played 9th Edition Warhammer because I didn't like the rules. I'll give 10th Edition a try, because it looks promising. Either way, though, I love Warhammer lore and I love Citadel miniatures, so I use the lore and miniatures in most of the other wargames I play, and it works exactly the way I want it to.
All images in this post copyright Games Workshop.