The Foundation series (in order of publication):

There are two ways to read this series. I would suggest reading them in the order that they were written, but you can read them in chronological order:

Prelude to Foundation, Forward the Foundation, Foundation, Foundation and Empire, Second Foundation, Foundation's Edge, Foundation and Earth.

The first three books form a trilogy, and are the core of the entire series (that is why I suggest reading them first). The two following the series are tightly bound, and really deal with one central story that moves away from the thrust of the original three. The two prequels serve to lend background information about the central themes, while attempting to tie in Asimov's robot series. All books should be readily availible at any library, but I did find Foundation and Earth hard to locate.