I keep wanting to try to make the Backrooms into an RPG mini-setting, but we already have Stygian Library and Vast in the Dark, so I'm not sure what I could do to top either of those.J.G. Ballard’s short story “Report on an Abandoned Station” turns out to be, almost exactly, “Ballard invents the Backrooms 40 years early”, and is tremendously creepy and cool in that very Ballardian way.
Side note. Thanks for mentioning the Stygian Library so often. I had the PDF but hadn't dove in. That's a fantastic book.I keep wanting to try to make the Backrooms into an RPG mini-setting, but we already have Stygian Library and Vast in the Dark, so I'm not sure what I could do to top either of those.
If you like fantasy, you might try Martha Wells's newest book. Name escapes me at the moment. She loves her mysterious megastructures.I got a sudden urge to read about encounters with mysterious megastructures, and have a pile of promising recommendation to re/read.
Strongly second the recommendation, even if megastructures aren't your thing (for me they're neither an attractor nor a repulsor). I know from firsthand experience you can enjoy it immensely, even if you miss like 90%+ of the Easter eggs in it.Piranesi by Susanna Clarke is about a mysterious megastructure. Recommend it.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.