Permutation City by Greg Egan. The central idea presented in this scifi novel is so compelling, I can't help but feel there is some truth in it. Also, I never feel I understood everything completely. Re reading helps clarify some new question every time.
What an existentially depressing book though, at least for me. It was wonderful, and worth reading more than once simply because it helps absorb more of its deeply complex aspects, but without a doubt it creates an unusual emotional impact.
