I finished the entire series by audiobook the day I left my last job.  It is taking a while to get this review written.  Just gathering all of the publishing data is proving to be a daunting task.  But I will get it all posted.  Note better reviewers than I have delivered more than I; see Wikipedia for better info.  This posting is my impressions of the series, nothing more.

The core of the Foundation Series is 7 books written over the span of 50 years.  The original Foundation Trilogy is widely considered among the greatest science fiction written and Asimov's greatest claim to fame.  Only the Robot stories have earned him higher fame, but the books themselves have not.  The Trilogy is considered required reading by and for any hard-core science fiction fan.  Asimov claimed to not have intended to write anything more of the series past the trilogy, but bowed to fan pressure to add more to the story.  The near thirty year hiatus certainly bears this out, though he never really "ended" the story with the establishment of the 2nd Galactic Empire.  He left us all guessing.

Update:  I don't think I am going to post the epic review I had planned.  It's just too big and would contain too many images if I used my normal review style.

The series is composed of the following books, telling the story of the great Galactic Empire, some 20k years hence:

Foundation Trilogy:  (Foundation, Foundation and Empire, Second Foundation)  These were the original stories written in the 1950s and are basically a collection of short stories that narrate the first half of the millenium long project to ease the transition between Galactic Empires and eliminate the 10,000 years of anarchy caused by the fall of the first Empire.  It was the first great expression of Science Fiction and has become part of the geek lexicon and pop culture forever.

Foundation's Edge:  This book was the first of the series to deal with only one main plotline revolving around a central character.  Golan Trevise, a member of the Foundation Council, is sent off into exile to covertly find the Second Foundation by overtly searching for Earth.  He does the first, nearly, but fails the second.  We learn of the true nature of the Second Foundation and the role it has in the series-wide plot.  This book was written nearly 30 years after the original trilogy, and it shows.  It is much more refined, but almost tedious in its occasional preaching subplots.

Foundation and Earth:  The conclusion of the Search for Earth storyline and a sequel to Foundation's Edge.  The long search for Earth reveals many incites into human nature, that truthfully may not have needed exploring.  The twist at the end ties together many loose ends of the original stories and converges Asimov's other major series, the Robots.  The book itself was great, if long.  I personally feel an aggressive edit would turn it into a excellent read.

Prelude to Foundation:  It is rumored that Asimov couldn't figure out how to continue the original storyline so decided to write a prequel.  Prelude is the telling of Seldon's arrival on Trantor and establishment of psychohistory.  Unfortunately, not one of the more exciting novels written, but you know what was going to happen to the main character ultimately anyway.  Still, some good plot twists and further melding of the primary Asimov franchises.

Forward the Foundation:  The last book ever written for the series as Isaac Asimov died soon after.  It was the continuation of the Seldon storyline including the period of his political office, establishment of the two Foundations and concluding where the very first book started.  A good read, but slightly disappointing in the same way that Prelude was.

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The Foundation series is one of the greatest epics in Science Fiction, in my opinion.  How far you read through it could be considered a measure of your geekness.  But of course, I have read it twice and all of the Robot series too, so I might be a touch prejudiced.  The Foundation Trilogy itself did really define modern Science Fiction and therefore should be read by everyone.