Diaspora - Greg Egan
The book starts with the birth of a child, a child that has been purposefully altered randomly to investigate the parameters of consciousness, but not a child in the flesh. You see, in this future, the vast majority of citizens live in one of many polises. These are smallish, nanotech computing environments that, through extensive sensor networks, create a virtual solar system. In essence, most people are uploaded into computing substrate. There are still fleshers though (citizens who have chosen to stay in biological forms, however altered through genetic manipulation they may be), as well as the Gleisner's who exist in robotic bodies.
The basic story goes that a nearby gamma ray burst makes all citizens realise their mortality, and the mortality of their civilization despite their relatively advanced technology. This prompts a diaspora, where copies of the polises are sent in numerous directions through space, in search of alternative systems to live in, and hopefully for answers as well.
The book was enjoyable, but at points the high dimensional physics got a bit hard to understand late at night, and could lead to some weird-ass dreams if you read it just before sleeping. Plus, if you get tired of the physics and put the book down half way through a theory, then trying to pick up from where you left off is frustrating.
Glad I read it though, 3/5 gamma ray bursts.