The Foundation series started off with a trilogy of three books – “Foundation” (1951), “Foundation and Empire” (1952), and “Second Foundation” (1953). Due to popular demand, Asimov continued the series with “Foundation’s Edge” (1982), “Foundation and Earth” (1986), and wrote two prequels “Prelude to Foundation” (1988) and “Forward the Foundation” (1992).

The series were continued by another trilogy popularly known as the “Second Foundation Trilogy” – Foundation's Fear by Gregory Benford, Foundation's Chaos by Greg Bear and Foundation's Triumph by David Brin.

Suggested order of reading is the same as above.

The basic theme of the Foundation Series is a vast interstellar galactic empire, which many believe is still in its heydays. But few, especially an old mathematician Hari Seldon through his knowledge of psychohistory, know that the empire is slowly and steadily declining and a thousand galactic years later will be followed by a dark period of suffering for the mankind, which will span over 30,000 galactic years.



The first of the three books – Foundation captures the story of its namesake, which Hari has helped set up as an anchor for humanity in those dark times. This is the story of Salvor Hardin and Hober Mallow, and how they successfully navigate through various Seldon Crises and ward off aggressive attempts of their belligerent neighbors to subjugate the Foundation and its capital Terminus.



The second book of the series – Foundation and Empire, continues on with the story with a focus on the Galactic Empire, which having had forgotten about Foundation is now beginning to see it as a threat. Though the Galactic Empire is in its death throes and has lost almost all control over its peripheries where Terminus is located, it is still a major force to reckon with. And along with this threat, there is the Mule, who is threatening the ultimate collapse of the Foundation along with Hari Seldon’s plans.

An interesting aspect of this series is that unlike books such as “A Song of Ice and Fire” by George R.R. Martin or “Wheel of Time” by Robert Jordan there is not a single battle scene described and yet it leaves the readers on the edge of their seat craving for more and that not in a bad way. The suspense leading to the climax and its aftermath, with a major emphasis on character building and dialogs between the lead protagonists is a hallmark of Asimov’s writing in the “Foundation” series.



The third book of the series - Second Foundation reveals the presence of another Foundation secretly set up by Hari Seldon to help guide the “First” Foundation. This is the story of Mule’s and eventually the First Foundation’s search for the Second Foundation and the conflict that ensues between them.



The Second Foundation has been destroyed and the First Foundation has emerged victorious. Was it predicted by Hari Seldon or has his plans been irrevocably damaged? Or is the Second Foundation still lurking out there, or is there some other mysterious third force subtly guiding events in the galaxy? Foundation’s Edge is the story of Golan Trevize who may have the fate of the whole galaxy resting on his shoulders.



In Foundation and Earth, Trevize along with Pelorat and Bliss attempt to seek answers by seeking the location of Earth, the knowledge of which has been systematically removed from everywhere, so much so that its name is considered an obscenity in many worlds outside the Foundation’s sphere of influence.



The first prequel, Prelude to Foundation, is a story about Seldon and his initial days on Trantor; about the resourceful Hummin and his accomplice Dors; the relationship between Seldon and Dors; and also about the enigmatic Demerzel and his secrets. Seldon arrives as a young mathematician on Trantor, head full of ideas on psychohistory but with no practical approach on making it a reality, and gets jostled between various political forces who wants to use him.



After writing the last book for the series – Forward the Foundation, Asimov himself died within weeks of writing Seldon’s death. The story is about Seldon, whom Asimov regarded as an alter ego, his life’s devotion, the all-consuming fire to develop psychohistory, and his joys and despairs shared with Dors Venabili, and ultimately his poignant death after the establishment of the Foundations.

Foundation’s Fear is set in the period covered in the early chapters of Forward the Foundation, while Foundation's Chaos covers the early chapters of Foundation. Foundation's Triumph covers various loose ends and describes the setting up of holographic messages to the Foundation.

All in all, foundation series is one of the epic science fiction series spanning over 500 standard galactic years, winning the one-time Hugo award for “Best All-Time Series” in 1966 beating even “Lord of the Rings” by J.R.R. Tolkien.

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Resources:
1. Asimov Home Page
2. Wikipedia - Isaac Asimov
3. Wikipedia - Foundation Series
4. Isaac Asimov's Book Reviews
5. Hugo Awards
6. Nebula Awards