Hey everyone! Is there any further reading that you could suggest for background lore on Baldur's Gate and it's surrounds? I'd like to get completely immersed for BG3

Well restriicting it to 5e material I'd suggest Sword Coast Adventurer's Guide.
Not about Baldur's Gate specifically, but Volo's Guide to Monsters exploring Monsters and Races common to the area like Yuan Ti, Orcs, Lizardfolk, Gobliniods, and more, also its Mindflayer section I think helped inspired Larian Studios their story choices.
Not in Baldur's Gate, in closest Major City is Waterdeep, which is detailed in Waterdeep: Dragonheist and its greatest Dungeon Undermountain is detailed in Waterdeep: Dungeon of the Mad Mage.
Mordienekian's Tome of Foes has a section on the Gith, who are likely appearing BG3 as a counter to the Mindflayers. Plus its has lore exploring the demihuman races of Halflings, Dwarves, Elves, Gnomes, and sort of Tieflings.
And finally Storm King's Thunder has a chapter with plothooks spread throughout Swordcoast Region.