Read the books Homeland and Sojourn (particularly the latter, though it only makes sense in the context of the first) by RA Salvatore.
They aren’t long books but they do a great job of showing what day to day life may have been like in the underdark. Those two books probably informed me of the Underdark as a setting and not just a big dungeon.
incidentally I’m not a massive Salvatore fan, I just think those particular books are excellent. The rest of the Drizzt books can take a long walk off a short pier.
If you have more time, the War of the Spider Queen books build on what was first expounded in the first books I recommended.
Doing a reread of WotSQ for my Drow campaign.
Using OotA as source material. Thought about running it. It's not good.
@Charlaquin you've basically nailed it about OotA.