I tend to recommend authors as opposed to individual books. I also like hard SF, so here are some suggestions.
For hard SF with a cyberpunk flavor, try William Gibson or Neal Stephenson. Gibson's Neuromancer trilogy is excellent. I love Snow Crash by Stephenson, and Cryptonomicon is good as well.
Sheri S Tepper writes an interesting brand of hard SF that's hard to describe. I'd recommend either Grass or Beauty as good novels to read by her.
Connie Willis has some time travel novels (Doomsday Book and To Say nothing of the Dog) that mix SF, romance and comedy. I love both of them.
If you've never read any of Terry Pratchett's Discworld novels, give them a try. If you've ever read Douglas Adams and liked his work, you'll like Pratchett as well.
Douglas Adams - if you've never read The Hitchhiker's Guide to the Galaxy, go out and do it now.
Neil Gaiman - excellent writer. Neverwhere is a particular favorite of mine. Lots of people recommend American Gods, but I prefer Neverwhere. He and Terry Pratchett wrote a book called Good Omens, which is a comedy about the apocalypse. It's great.
Douglas Preston and Lincoln Child - they write thrillers with a SF twist to them, and are best known for Relic. There's a whole series featuring characters from that novel, and they're all pretty good thrillers. These guys are good rainy day reading.