I just finished Gregory Benford's 1980 Nebula Winner Timescape. I will review it over at Jupiter Three, but let me just say, it definitely qualifies as Hard Sci Fi.
The author is a physicist, and his handling of the physics of time and space was deep enough to boggle my mind. OK, maybe not a stretch, but still.
The end of the copy I have has an excellent essay by Susan Stone Blackburn entitled Science Meets Literature, which I found to be just as interesting as the book. The essay was published in 1988, and helped me to understand the book after I read it. (Warning - the essay includes spoilers, so don't read it first, no matter how helpful it would be!)
Enough said here; but if you are looking for a good example of Hard Science Fiction, this is an excellent sample!