A Fire Upon the Deep is a big old high concept science fiction novel, written by Vernor Vinge and first published in 1992. It won the Hugo Award for Best Novel in 1993.
Vinge works (here, anyway) in the grand old hard SF tradition of making some interesting assumptions about physics, imagining how those might play out in terms of technology and life in general, and then writing a story which is inextricably bound up with all that stuff. It's all about imagination, folks, that and unity of purpose: If there's anything much in there that's just optional or arbitrary window dressing, you've got a dog on your hands. When it's done right, all things appear to proceed inevitably from the basic premise. In a way, hard SF is like detective fiction: Everything has to hold together logically. It's got an advantage over mysteries in that the defining characteristics of the genre aren't so cramped: "It's about a crime" limits an author's options much more than "it's about what things might be like if the laws of physics permitted such-and-such." You can spin a greater variety of situations and conflicts out of the latter.
The major premise in Fire is this: Speaking somewhat loosely, Mr. Vinge suggests that natural law is affected by how much mass there is near by (for values of "near by" that are measured in light years, not subway stations). When you're too close to a galactic core, you can't travel faster than the speed of light, and you can't do a lot of other things either. No AI, for example. The farther out you get, the easier it is to make neat stuff. Mostly, the phenomenon affects information (Vinge is a CS prof, by the way). The region at the very center of the Milky Way galaxy is called "the Unthinking Depths": You can't even have sentient life down there. In the "Slow Zone", it's possible for organic entities to think (it looks like that's where we on Earth are stuck). In "the Beyond", pretty much anything is possible. Nobody has a clue what's going on farther out, in "the Transcend", because anything that can survive out there has advanced to the point where it can no longer communicate with the dopey slobs farther in. This is a classic Big Science Fiction Idea. In the right hands, it throws off implications and littler ideas in all directions.
The plot involves a murderous and power-hungry AI, inactive for billions of years, which comes to life in the computer systems of a human colony that tried to establish itself a little too far "up" in the Beyond. It wipes them out and goes about taking over the rest of the Beyond. A variety of aliens and humans spend the next 600-odd pages trying to put the kibosh on it.
There's a nifty subplot involving planet-bound wolflike creatures in the low Beyond. They're group organisms. Thinking "persons" are groups of four to six of these creatures, who individually are nearly brainless. The individuals in a given group share their thoughts using sounds. Lucky them, they're far enough "up" from the galactic core that a stunt like that can actually work. When the members of an individual are scattered too far apart, they get disoriented. After a battle, temporary "individuals" will form out of the survivors of groups who lost most too many members to function as thinking beings. The implications are endless; what does identity really mean, in that situation? When half of me can merge with one third of you and a quarter of that guy over there, who's "me"? Who are "you"? It's fun stuff. Vinge again devotes a lot of time to exploring all the implications, while also keeping an adventure novel plot moving along at a smart pace -- and while obeying the other Law of Hard SF: The major conflicts concerning the "wolf group entities" all proceed from, or are heavily influenced by, their nature.
I've barely scratched the surface of either of the two "Big SF Ideas" here.
When you think about it, hard SF sounds ridiculous on paper: "Okay! Let's write an adventure novel about physics!" Really, who the hell would want to read such a thing, much less go and write one? Why throw those two things together? Well, what we've got here is an existence proof to the effect that hard SF can be very much worth writing, and a blast to read.
Oh, yeah, the prose: Mr. Vinge is no James Joyce. I think the word is "serviceable". Characterization? Uh, yeah, there's some of that in there somewhere. You can't have everything.