Seveneves - Neal Stephenson. On space exploration incentivized by an earth-wide catastrophic event.
The Culture Series - Ian Banks. Especially: The Player of Games. A series of space operas about the lives of technologically advanced civilizations.
The Last Question - Isaac Asimov. On solving entropy.
Blue Mars, Red Mars, Green Mars - Kim Stanely Robinson. A series of books on civilizational problems on the path to colonizing Mars.
Explore the Cosmos in 10 Classical Space Operas - Alan Brown. A link list of space operas.
The Moon is a Harsh Mistress - Robert. A. Heinlein. On political struggles on planetary colonies.
The Dispossessed: An Ambiguous Utopia - Ursula K. LeGuin. On choosing amongst anarchism and other political regimes that govern different planets.