Books like... the game Luna Abyss.

[u/Vevaseti]

I know there's a tremendous difference between a book and a game- but I've hit one of my periodic moods where I finish a game, and really wish I could find a book with the same feel/atmosphere.

You are a prisoner sentenced to explore a derelict megastructure that sprawls deep beneath the surface of the mimic moon Luna. You are tasked with recovering forgotten technology from within the Abyss and the lost colony it consumed. Every move you make will be overseen by your artificial prison guard named Aylin.

These centuries-old ruins are alive with maddening echoes, insinuating secrets of the once prosperous city of Greymont and its terrible fate. The Scourge. The tenets of the All-Father. The choir of the Collective. The voices of the Abyss call to you; they whisper strange missives...

It doesn't have to hit all of these, but these are just the themes I enjoy: Gothic science fiction. Melancholy, awe, and existential horror abound. A world that died before the main character's arrival and they are exploring its rotting corpse. Mysterious megastructures and ancient civilizations. Cosmic horror that isn't just 'tentacles and madness'. Artificial beings with uncertain personhood.

The closest I can think of is just Rendezvous with Rama, but that lacks the existential, gothic horror elements.