If you’re interested in reading any of the books, I would recommend starting with the Horus Heresy novels. There are a lot of them, and some are better than others, but the first three are a good place to start. They’re sort of the bigining of everything and lay a good foundation for the lore moving forward. The Lexicanum is a good wiki to rabbit hole on if you just want to ingest raw lore.
Makes sense, it kind of sets the ground work for the whole universe.
And no problem