Sorry for the delay in responding, life has been hectic. I’d guess the Horus Heresy is a pretty typical entry that would be recommended. While its set in the 30k era, it really sets the stage for why of how things got so bad in the rest of the 40k series. if you’re not looking to slog through ~60 books to read through just the Horus Heresy, I’d recommend checking out the 40k wiki: Here’s a link to the timeline of events. I’d recommend just following links around for things that sound interesting.