Agreed about A Fire Upon The Deep, optionally followed by A Deepness In The Sky. Those are classically styled hard science fiction novels with spaceships and aliens. But much more thoughtful than you might expect from a typical spaceships and aliens scifi novel.
If you're looking for something more germane to present concerns, Rainbows End is about a near future where people's interaction with the world is mediated by augmented reality and various forces are fighting over access to information.
And since I haven't seen it mentioned here yet, Marooned In Realtime is also really good.
But if you're looking for a single book, then you won't go wrong with A Fire Upon The Deep.
I disagree with the notion that A Deepness in the Sky requires having read A Fire Upon the Deep. In fact, I would go so far as to say that each ends with an open question that is answered by the other, so that no matter which one you read first you will discover the answer in the second.
Seconded, and it touches on the key themes he developed later. I love how a throwaway plot element became a central part of an unrelated novel later, like he had more ideas than he had time to fully explain.
