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Sorry if the wording of the title is a bit confusing... Lemme explain.

A lot of the pre-Imperial era (Age of Technology, Age of Strife) and early Imperial era (Great Crusade) is shrouded in myth. For a time, even the Horus Heresy was just the broad strokes, though after 50+ books, more words have been written about it than any other 40k event.

What I'm curious about is, what's the earliest piece of lore (be it a novel, or a mere paragraph in a codex) set in the 40k universe and involving humans, the Imperium, or what would one day become the Imperium? (I'm not talking about ancient races like Eldar and Necrons.)

When I talk about detailed lore, I mean that there's dialogue, and/or characters performing small actions ("he gave him a hard stare", "he shrugged") as opposed to legendary deeds.

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Valdor: Birth of the Imperium is probably the earliest set novel within the 40k universe concerning the Imperium that fits your requirements.
There are snippets from other novels such as Belisarius Cawl: The Great Work where Cawl interacts with the Emperor through a weird dream/vision sequence but apart from that I don't think there's much else.
The earliest novel set in the 40k timeline would be The Infinite and the Divine but it's about Trazyn and Orikan.

For understanding pre-Dark Age Humanity, I’d strongly recommend the Votann Codex.

No there’s no character dialogue as such, but what the Kin know of their origins can be interpreted a number of ways, including as a cloned slave race sent out by mega corporations.

Some of the Horus Heresy novels following the perpetual storyline recount events in the distant past, including what is implied to be the defeat of the Tower of Babel so assuming that is ancient Babylon, then somewhere around the 6th century BCE.

Aash wrote:
Some of the Horus Heresy novels following the perpetual storyline recount events in the distant past, including what is implied to be the defeat of the Tower of Babel so assuming that is ancient Babylon, then somewhere around the 6th century BCE.

Closer to seventh century, but yes.