The City and the Stars

Temporally and thematically epic. One one level it's about a 'young' man, Alvin, discovering a way to transit from his centrally programmed hyper-technoligized city, Diaspar, to an ancient Athenian-style city-state known as Lys. Along the way Alvin and his Lysian friend Hilvar discover a massive crater with an ebony-like surface that plays host to a robot that's billions of years old, and holds the key to an age where Earth still had its oceans.

It's a metaphorical SF novel about civilizational death. Obelisks included because it's Clarke. It's nicely paced. Sub-250 pages (at least the 1980s Gollanczs PB edition I read was). The planetary exploration in the latter half, an animal called the Polyp, and Diaspar's design made for really interesting reading.