I can give you plenty of advice about science fiction. Many of my recommendations here will appeal to folks that don't normally read much sci-fi, though I'll make a note if there are exceptions.
Another vote for "Ender's Game" and "Speaker for the Dead" by Orson Scott Card. Unfortunately, you'll get so wrapped up in those two that you'll be forced to read "Xenocide" and "Children of the Mind" too. They aren't as good. BTW, read "Speaker for the Dead" even if you're one of those rare people who doesn't really enjoy "Ender's Game." They're very different, and I actually liked "Speaker" best. These books wouldn't appeal as much to someone who isn't into sci-fi.
The Hitchhiker's Guide to the Galaxy 5-book "trilogy" by Douglas Adams. It's hard to believe there are people in the world that haven't read these, but semi-reliable sources insist it's true.
"Dirk Gently's Holistic Detective Agency" also by Douglas Adams, followed by the sequel "The Long Dark Teatime of the Soul." BTW, I can actually explain what happened at the end of "Dirk" if anyone's interested... you have to have a working knowledge of Coleridge to grasp what happened.
"Birthright: The Book of Man" by Mike Resnick. GREAT novel. Nothing else he wrote since then quite measures up, unless you like the "western set in outer space" type sci-fi, which he's very good at.
How about a few short story collections...
"R is for Rocket" and "The Martian Chronicles" by Ray Bradbury. Both excellent. "Martian Chronicles" actually works as a novel, with each short story loosely connected to the others in a larger story. Come to think of it, "Birthright: The Book of Man" (recommended above) is set up the same way.
"The Persistence of Vison," "The Barbie Murders," and "Blue Champagne" by John Varley. Varley's someone I wouldn't recommend unless you're a sci-fi kind of person. But he's an absolutely EXCELLENT writer, and his short stories are top-notch. Many (though not all) of them are set in a really interesting (IMO) future history in which humankind has been banished from Earth by mysterious invaders. Humans are now forced to live on the moon, Mercury, Mars, etc...
A couple warnings about Varley's work, though. 1. Not for kids. Sex is not really the point of the stories, but there's enough there, and not all of it is 'normal' (for lack of a better word). 2. It can be somewhat confusing figuring out what's happening at the beginnings of some of his stories. Stick with it and it will all make sense.
Okay, enough for now. You can thank me later.
Russ "The Peripatetic Audiophile"
Edit: Oh! I almost forgot. READ "Jurassic Park" by Michael Crichton. Much better than the movie, IMO. Makes me wish James Cameron had directed the movie rather than Spielberg. The novel always struck me as a James Cameron kind of story.
