Lady of Mazes by Karl Schroeder, a far future hard science fiction novel. My impressions of the book are pretty normal for me and this sort of story: I think the ideas and concepts are really fantastic and thought worthy but the story itself ends up being somewhat flat.
Many books in this genre end up presenting remarkable scenarios but not explaining them well enough for people to really understand what is going on. Not explaining everything in a book is fine as the reader is very much expected to imagine much of the story themselves to fill in the gaps but when the world is so utterly alien and makes so little sense from a 2011 perspective it becomes infeasible to do so in any way that feels satisfying. In this case the world is filled with many (nearly infinite?) numbers of different 'manifolds' which are essentially like versions of reality overlapping the same locations in space and time. We are given a very basic primer on how these work but it is sketchy enough and violates all known physics enough that figuring out the bits of the story that aren't explicitly laid out is impossible. Once hundreds of people can occupy the same space simultaneously without even being aware of one another it is hard to imagine exactly how the rest of the story might 'logically' fall out.
The best bit about this book though is the idea of animas. This is the name given to a software construct designed to perfectly emulate a human. The characters live in a society where everyone creates many animas of themselves so that their friends (or enemies!) can constantly have access to them regardless of what their physical bodies might be doing. This is a really intriguing extension of the current trend toward having constant communication between people; cell phones and texting have meant that getting answers and social interaction have become quicker and easier and animas take it to the next level. You want to know what Joe thinks of a topic? Just ask the anima of him you keep on hand at all times! The societal implications of these constructs are outlined a little bit in the book too and in the story they challenge others to duels, make dates and do other things that the original meat person is expected to answer for. It is made clear that if you create an anima and use all of the tremendous functionality involved you are entirely responsible for what it does since it is in fact doing what you would do in that same situation. If you don't want to accept that responsibility, don't make an anima... but everyone will find you intolerably rude since you won't be able to hold 10 conversations simultaneously like everyone else.
Obviously the technology to create software simulations of a person that are so accurate that no one can tell the difference is far beyond us today but it certainly isn't beyond the realm of possibility. Human brains are deadly complicated but computers continue to improve in capacity exponentially and eventually we must be able to find a way to cram a brain sized amount of data into a computer. Guessing about social constructs hundreds of years in the future is purely for fun of course since people of 1980 wouldn't have had any clue about online forum behaviour today and the rate of change is accelerating but I do think that Schroeder's version of future communication is far more realistic than most other science fiction I have read. It feels much more like an extension of current trends to the nth degree than the usual '2011 man with a ray gun and FLT spaceships' science fiction.