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I have been getting into 40k lore recently, especially that of the Death Guard and the Adeptus Mechanicus. I skimmed their 8th edition codexes online after hearing about all of the fluff they contained and I really enjoyed them, as I loved the details they fleshed out about units and subfactions. I will probably end up buying hard copies of the 8th edition codexes, as I prefer paper media to digital. I’m wondering if there are any other expansive sources of fluff (like other older codexes) for the factions I’m interested in (other than wikis and the like). I’d love to hear about them and if you think they would be worth it for a newbie getting into 40k lore to purchase. Thanks in advance!

For Death Guard, I would first read Flight of the Eisenstein, followed by the Dark Imperium Books and then The Lords of Silence.

Lords of Silence is really good. Flight of the Eisenstein and Dark Imperium didn't really appeal to me as a fan of Chaos Death Guard, but they're nevertheless good for an overall impression.
The Burried Dagger shows the fall of DG to Nurgle, but I haven't read it yet.
If you can get hold of it the Forgeworld 30K book one Betrayal describes a lot of their origins, I'm not sure where that fluff is available nowadays. The basic facts you can find in lexicanum as well of course, but these 30K books also have an outstanding writing style.

The buried dagger is really good as it explains the origin of the death guard's final fall to nurgle. But you might find it a bit difficult to follow as it's the last book of the Horus heresy before the siege of terra starts. There's a lot of characters and references to things that happened in the rest of the series

The core rulebook is IMO a great place for Warhammer lore, giving a solid rundown of the setting and where it's at.