All of these are pretty much hard sci fi (hard scifi = scientifically correct sci-fi, everyone thinks I'm showing off by saying hard lol. It's an actual genre just google hard sci-fi). Mostly about space exploration, biology, life, economy, etc... in space.
1) 2001: A Space Odyssey (a classic)
2) Ringworld
3) Eon
4) The Mote in God's Eye
5) The Ringworld Engineers
6) Rendezvous With Rama
7) Dune
8) Lucifer's Hammer
9) A fire Upon Deep
10) A deepness in the sky.
Honorable Mentions: The rest of the space odyssey series, Rama II, The Ringworld Throne, Hyperion, The Fall of Hyperion
1) 2001: A Space Odyssey (a classic)
2) Ringworld
3) Eon
4) The Mote in God's Eye
5) The Ringworld Engineers
6) Rendezvous With Rama
7) Dune
8) Lucifer's Hammer
9) A fire Upon Deep
10) A deepness in the sky.
Honorable Mentions: The rest of the space odyssey series, Rama II, The Ringworld Throne, Hyperion, The Fall of Hyperion