Hard SF?
Rendevous With Rama and the
Odyssey series by Arthur C Clarke (well, anything by ACC, actually).
The
Foundation and
Robot series by Isaac Asimov.
Friday by Robert Heinlein, as well as just about anything else he wrote.
Ringworld by Larry Niven (and the sequels, if you like the original).
Across Realtime by Vernor Vinge, especially "Marooned In Realtime."
Chindi by Jack McDevitt, and the rest of the Patricia Hutchins series as well.
Telzey Amberdon by James Schmitz, and any other Hub-related stories as well.