After reading the sequel to this book, Children of the Sky, which felt tepid to me, I reread this novel to see if it was really as good as I remembered. And it is. This was my introduction to Vernor Vinge decades ago and the second reading was a stronger experience than the first. This novel introduces the notion of the Unthinking Depths, the Slow Zone, the Beyond, and the Transcend, which are the Zones of Thought that give the series its title. The idea is one of the best in science fiction, that the laws of physics are zoned in a hierarchical way, not necessarily by nature but due to action of one or more of the transcendent races at the tip-top of the galactic food chain. A human colony at the Top of the Beyond has crossed over into the Low Transcend to do work on an archive that turns out to contain something quite nasty, and this sets in motion a story that arcs through thousands of light years when the human scientists flee the archive and their remnants end up on a world inhabited by packs of doglike creatures who achieve sentience in groups. The ideas are amazing and entertaining and they expand and expand in Vinge’s careful story. The medieval dog-world (Tine’s World) contrasts with the high powered cultures of the Beyond in a marriage that grounds the high-flown ideas about Powers and Transcendence and each side of the story balances the other. I loved this book the first time but I don’t think I absorbed nearly as much of it as I did after a second reading. It is one of the best examples of the new space operas that started to appear late in the last century, and the world building – well, one might as well call it galaxy-building – is superlative. If you are a science fiction reader you probably already know about this book but if you don’t, you really should crack it and take it all in.