I really wanted to like this book more than I ultimately did. It's chock-full of ideas -- the airspheres, interactions between civilizations of vastly different ages, explorations of different ways societies might deal with the digitization of consciousness, and much more. Some of the individual dialogues were deeply thought-provoking, especially Ziller's discussion of creativity with the Hub. Unfortunately the novel as a whole just didn't hang together. The central plot seemed contrived and implausible, with multiple gaping holes, and the trip to the airsphere entirely pointless. The "reveal" of an ultimately unnamed and unknown enemy of the Culture capable of putting the "attack" together felt more lazy than anything else. The Culture universe remains a fascinating place none the less.