Also, the core rulebooks of GW's tabletop game. I think each edition included some sort of introduction, sometimes with more, sometimes less detail, but they all provide a good introduction as they are meant as the starting point for people new to the franchise, whereas the novels are intended for existing fans that have already consumed the aforementioned information.

That's what I meant, actually

But yeah, some edition had it more or less. But anyway, in my opinion, 40k is a universe that you win at learning at a slow pace. But this is a problem when you are a game master....

It is just so hard to bring in people who aren't familiar with WH40k, as reading a big stack of books is the only way to get introduced.

I think, in addition to the brief overview of the 40Kverse that most WH40KRP core rulebooks contain, a 40K glossary defining many of the universe-specific terms would be a very helpful 'pimer' for newcomers.

That's what I meant, actually

Oh! Thought you were referring to the RPG books, as they too talk about the background.. ^^'

And I agree about the learning thing, of course. It helps a little if you start your game with a setting and characters that aren't defined anywhere, and 40k definitely allows for a wide variety of "local flavour". Though there are certainly some things that are "critical" general knowledge ...

I think, in addition to the brief overview of the 40Kverse that most WH40KRP core rulebooks contain, a 40K glossary defining many of the universe-specific terms would be a very helpful 'pimer' for newcomers.

Well, a lot of novel authors use their own terms, invented and deployed exclusively by that one author ... but if it helps, I noticed this the other day.

If you can get your hands on it, Black Library's Inquisition Illustrated Guide is also a pretty good resource for inspiration. It focuses mostly on Abnett's novels (imho it would not have been necessary to give even the minor characters of his books their own entries ..), but for a Dark Heresy campaign, there's a lot of stuff you could steal. Nothing as basic as explaining what an autoquill is, but it does explain what things like an auto-seance or cyber-mastiffs are. It also goes into more detail regarding psy-levels, Inquisitorial seals and trials, or various cults and sects.

It is just so hard to bring in people who aren't familiar with WH40k, as reading a big stack of books is the only way to get introduced.

Well, one of the nice things about the 40K universe is that there's very little that a player needs to know in order to get started. Most citizens of the Imperium know nothing of the Warp, or Tyranids, or the Dark Age of Technology, or anything else that isn't of immediate relevance to their lives. Most have no idea what lies beyond their own homeworld; maybe even their own city; maybe even their own hab-block.

When I started up my DH campaign, only one of my seven players knew anything substantial about the 40K universe. Most of them had seen the David Lynch film version of Dune, which at least gave them some kind of frame of reference for the technological aesthetics. I explained to them about the Ecclesiarchy and how everyone was expected to worship the god-emperor of mankind (or else be burninated), but that the particulars of said worship varied from planet to planet. At that point, I simply asked each of them what kind of place they wanted to be from and what kind of early life they wanted to have. Based on that, I would select an appropriate homeworld for each player and we would fill in the particulars together.

And that was all they really needed. All the other grand facets of the universe like Chaos and the Eldar and Space Marines; the players are learning about them as their characters learn about them.

There are a lot of in-universe rules and expectations for different people and places, though. While Only War is an easy introduction (WWII in space for a frame of reference) more nebulous concepts like what an Inquisitorial Acolyte is or what sort of things a Rogue Trader can do are harder to explain to someone who doesn't know 40k as well. The setting is influenced a lot by Dune, by the Dark Ages, by Catholicism, etc., but the interplay between all the different themes isn't exactly intuitive.