Pattern Recognition by William Gibson. Really, anything by Gibson is worth reading multiple times, simply for the great writing style. Pattern Recognition also does something special: have you really ever /thought/ about brands and advertising? Lots of commentary on culture, memetic fashion, and what it means. Recursion by Tony Ballantyne is a book that I didn't actually like all that much, but reread anyways. It covers a lot of topics I love (global AIs, Von Neumann Machines, tulpas, trusting trust, and a totalitarian utopia). The plot leaves a lot to be desired, but the concepts are worth it.
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