I, Robot
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This collection of short stories contains the history of the development of the “positronic” brain, which is a precondition for the existence of the robots in the Robot series of novels.  Several of the stories also explain Asimov’s famous “three laws of robotics”.  All of the stories are enjoyable and some are humorous.  Not that the book has nothing to do with the annoying move with the same name.

Second Foundation
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The concluding novel of the original Foundation trilogy.  Once you’ve read the first two you will want to learn if the Second Foundation can defeat the Mule and complete Seldon’s mission.  Highly entertaining.

Foundation and Empire
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The second novel in the original Foundation series should be read immediately after the first.  There are startling surprises in the trajectory of the Foundation.  Asimov gives the impression that his blueprints are based upon real history.

Foundation
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The first novel in the original Foundation trilogy is one of my favorite books.  This is the best novel to start with when reading the series as it sets the stage for all of the other books including the prequels.  It tracks the rise of the Foundation from the psycho historians through the merchant princes.  Asimov offers interesting solutions to the problems faced by fledgling Foundation.