(another review recycled from Goodreads)
This is one of my favorite books - a near-perfect hard SF novel, with fascinating concepts, interesting aliens, and evil (yet competent) villains.
It takes place in a complicated universe where humanity has traveled among the stars for thousands of years, but only slower than light, and technology though advanced, never produced breakthroughs like immortality, nanotechnology or true artificial intelligence, and no high-tech civilization seems to last more than the better part of a thousand years or so (humanity has become extinct on Earth and the planet has been resettled more than once by the time the book begins). People have grown accustomed to the limitations that apparently exist (the reasons behind these limits are explained in the earlier book A Fire Upon the Deep), and interstellar traders thrive by bringing interesting primitive artifacts to advanced civilizations, and a range of advanced tech to less-advanced planets; the traders even transmit technological information free of charge, so that when they arrive at a redeveloping world, the traders’ language and technology standards are likely to be in use. Computer systems contain solutions to all sorts of problems, if you know how to pull them out of the ancient archives - and “software archeologist” is a recognized and respected profession (it turns out the time system used by the traders takes as its zero point, the UNIX start time!). In one lengthy subplot the reader sees how this trading culture was invented, and how it did not reach the end state its inventor desired - and due to time dilation and coldsleep, the inventor is still around to resent this.
But all this is mere background - there are two more major plot elements: humanity’s first encounter with an alien race - one that is reaching an advanced state for the first time, in a very unusual environment, and a rising human civilization that has invented a new method of slavery, and has a fierce desire to expand. Vinge brings all these elements together in a terrific story which rewards rereading.