Diaspora is another masterpiece from Greg Egan. The premise: a future where humanity has split into fleshers (biological humans), gleisners (humans who have uploaded themselves into robotic bodies), and purely digital consciousnesses. Rather than being about the sociocultural interactions between these factions (like Bladerunner, or Asimov's I Robot), Diaspora is much more about the long term progress of humanity – how we might be able to overcome the challenges of an unforgiving universe.
For anyone remotely interested in AI, I would highly recommend that you set aside 15 minutes with a cup of tea to read the first chapter (which has been published as a standalone short story called Orphanogenesis). It is an incredible take on AI, aiming to capture the qualia of learning from the perspective of a "conscious AI", which makes it a nice complement to the factual accounts in a book like How to Create a Mind (or the work of Marvin Minsky).
There is a big "but". While I have no intention of gatekeeping Egan's work, my honest perspective is that this book would be far less enjoyable for someone without a university-level understanding of (astro)physics. Without giving away too much, one of the plot points is an unstable astrophysical system; unlike other authors who might cheat and say "the system was unstable", Egan (through the eyes of a character) explores what the telescopes / gravitational wave detectors would actually show if the system were indeed unstable, and tries to hypothesise physical mechanisms. Another motif is Kozuch theory, a (fictional) grand-unified theory (combining general relativity and quantum mechanics), which Egan verbally sketches out.
This is what makes Egan unique (and why I find his books so hard to put down), but it's a significant barrier to entry (Diaspora is tricky even by Egan standards – try Permutation City for something marginally more accessible). Despite learning General Relativity at university, I still found some of the discussions about manifold curvature tough to follow. I suppose one could enjoy Diaspora without following the physics, but it somewhat defeats the point – better to just read PK Dick in that case (if you do want a taste of Egan, his short stories are easier).
All this is highly unfortunate because the concepts are mind-blowing. From the philosophical questions about what it means to be conscious, to the "big-picture" questions about humanity's place in the universe, every page is a delight. I was disappointed with the ending and was initially tempted to notch down my review from 5 stars to 4 stars. On reflection, I realised that my disappointment was less to do with the resolution of the plot and more to do with the fact that I, as a reader, could no longer inhabit the world of the book. This sentiment is incompatible with my pessimistic view of humanity's long term potential (see my review to The Precipice). Though I am yet to internally resolve this contradiction, this may end up being one of the few Egan works that has improved my outlook for humanity (most have a distinct nihilist/antihumanist flair).