This is an amazing piece of science fiction, and a wonderful idea-book. It is also a modern classic, recipient of the Hugo, and nominated for about every other award; a title I had wanted to read for some time.
In short, A Fire Upon the Deep is the story of how a party of human explorers awakens an ancient AI and has to flee to a backwater planet in the technological slow-zone where development currently is at a medieval state. It is also the tale of the rescue party sent to help them, and about a very particular human created by godlike technical entities. Above all however it is a story about what it means to be intelligent, and to be conscious. A Fire Upon the Deep is considered Singularity-SF I gather, but Vinge beautifully creates his Zones of Thought to control the concept, and are thus free to draw parallels between the post singularity depths and 'the slow zones'; each a model of consciousness, one echoing the other. As this is something I have thought more and more about over the last few years, I found the book very speculative and interesting. Moreover, I had created simple narratives, simulations, and scenarios where consciousness happens on crowds. To a computing scientist this seems to come natural, and yet somehow naively it seemed novel to me at the time. Vinge (a mathematician and computer scientist) wrote A Fire Upon the Deep over twenty years ago, and his Trines are exactly that.
A highly recommended book.