I wouldn't say I'm new to the lore and universe of 40k but I'm definitely not deep into it. I just read the books I find interesting and other than that have a vague broad understanding/knowledge of like the Horus Heresy and most factions.
Ive heard that way way back Space Marines were roided up criminals or something like that? I think I've heard that Russ was originally not a Primarch? All that is super interesting to me so if anyone has any old lore like that to share please do!
Also I wanted to ask when would you say the shift to what became "modern" 40k lore started and also if there are sources where I can read about the super old lore above?
Edit: Thank you so much to everyone who commented and gave suggestions on early stuff to read! I was under the impression that early 40k was a bit more wacky and very different from what we have now, but after reading throuyh some the first few editions I've learned that a lot of that was exaggerated and most of the basic lore was set up by 2nd edition at the latest.
Im definitely gonna read through as much of the old lore as I can in the morning, Id like to see the beginning of what we have today and see how things and characters evolved over time.