The Heresy has been an ongoing background to 40K since its inception in some way or form, and there is a lot of information (and misinformation) going on, regarding things holding over from previous editions fluff.
The current release of stuff has discounted that previous fluff, although possibly made comments regarding it as a #TBT to the vets who remember some of the older stuff. As such, I believe the older stuff is now blowing in the wind, other than to be used an inspiration - and we know GW loves its retcons.
So, where does that leave us with Heresy stuff?
Well, we have 3 full "campaign" books from Forge World, and two "gaming" consolidatory books, backed up by 25 novels, (not including the compilations) and its numerous shorts. This provides us with an absolute ton of information of characters, not only of the legions themselves, but the auxiliary forces serving with them. Most importantly, in my mind, is the characterisation of the Primarchs, not to mention the Audio exclusives (grumble, grumble, I'm-not-7-years-old-ffs, grumble).
I have an idea of stuff I'd like to do with it, but first, is there a possibility we can get a bit of a timeline going on, with regards to the novels?
Now, that's a lot of stuff to be working through, and doesn't include anything from the 40K setting telling us "what we know" of the 30K setting. Is anyone willing to help with this first task?