Isaac Asimov's robot series (start with "I, Robot" and "The Caves of Steel") is good, as is his Foundation series. Ender's Game is also good; I've heard other people complain about the other novels in that universe (Speaker for the Dead and sequels, Ender's Shadow and sequels), but I liked them fine. Cryptonomicon is rich, fascinating, and entertaining. Dune is rich and pretty fascinating. Heinlein's "Stranger in a Strange Land" and "Time Enough for Love" are somewhat scandalous, but I thoroughly enjoyed them. Outside science fiction: P.G. Wodehouse's Jeeves and Wooster novels, as well as what of his other books I've read, are hilarious and brilliant; I would suggest "Right Ho, Jeeves" as a starting point.
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