I read this deeply immersive book, set in far-far future Earth.
Humans had reached immense tech heights, but had lost all their technological knowledge and were regressed to medieval-esque lifestyles. There were ruins of future-tech everywhere, floating cities where the remnants of the overclass still lived, supplied by a handful of shuttles that brought them food from the people living on the ground.
There was an orbiting city where some inhuman being lived and it took human sacrifices up to consume.
There was a system of giant towers (shaped like the Eiffel tower) stretching from Asia to Europe as a cable car transport system.
The main protagonist goes on a journey to rediscover the keys to all the forgotten tech. In the end he finds a forgotten library which uploads all the knowledge of the lost civlisation into his mind.
Any ideas? I'm 90% sure I didn't just dream that I read this.