Prelude to Foundation & Forward the Foundation

Now, after having read the Foundation prequels as well, I would say that one might be better off just going through the original trilogy and leaving it there. You won't get a conclusion to Seldon plan (though how much do you get even with the sequels?) but you will keep the series separated from the 0th Law of Robotics. The trilogy will stay as a centuries spanning galactic struggle just among humans. These two prequels do at least bring back the Encyclopedia Galactica epigraphs I missed in the sequels.

Foundation and Earth

Foundation and Earth picks up where Foundation's Edge left off: Councilman Golan Trevize with his uncanny ability to always make the right choice decided that Gaia -- a superorganism planet where all things are part of a group consciousness on some level -- should be the eventual future of the galaxy instead of the First or Second Foundation. Gaia removed the memories of the planet's existence from the Foundation people involved in the stand-off at the end of the novel and everyone went back to their lives except for Golan and his historian friend Janov Pelorat.

Foundation's Edge (and about the Foundation series in general)

Second Foundation by Isaac Asimov was the only Foundation book I had read for the longest time. I have read it twice or thrice in fact as it can be found in my parents' bookshelf. Recently I figured I should maybe read the rest of the series.