I read a lot of sci-fi, so I'll give some suggestions in that genre:

Isaac Asimov - the "Grand Master of Scifi"
The Foundation Series
A little dated (written in the '50s), but very good nonetheless

Alastair Reynolds - writes good "hard scifi," i.e. based on modern scientific principles. He has a phd. in Astronomy I believe.
Revelation Space (and sequels Redemption Ark and Absolution Gap)

Robert Heinlein - another "Grand Master of Scifi," though I prefer Isaac Asimov. Heinlein work is a little uneven sometimes (as in I've read some books that I really didn't enjoy), but he can do great work when he wants to.
The Moon is a Harsh Mistress, Starship Troopers, and Stranger in a Strange Land (which I'm reading right now) are very good.