[KellyAdmirer]

Isaac Asimov's Foundation series is classic, especially the early volumes before he started getting politically correct. Lots of influences on later sci-fi.

[pro libertate]

Isaac Asimov's Foundation series is terrific. I also recommend the novel, "The Mote in God's Eye," by Larry Niven and Jerry Pournelle.

Other favorites:
"A Canticle for Leibowitz," by Walter M. Miller, Jr.
A number of stories from Philip K. Dick
"To Your Scattered Bodies Go," by Philip Jose Farmer
"Stranger in a Strange Land," by Robert A. Heinlein
Several short stories by Poul Anderson
and for light-hearted reading the Thomas Covenant series by
Stephen R. Donaldson.