The trilogy of books Foundation, Foundation and Empire and Second Foundation are among the best Asimov ever wrote among his science-fiction novels and among his most influential. I've read it was based on Gibbon's Rise and Fall of the Roman Empire. Only this empire spans the galaxy. Hari Seldon predicts through "psychohistory" the empire will fall within 300 years and establishes a foundation to manipulate history to shorten the dark ages that will follow. The fascination is seeing how all that plays out, especially in the first book. Foundation and Empire is less episodic than the first and features one of Asimov's most complex and compelling characters, "the Mule," as well as a strong female character, Bayta, and a clever twist. Second Foundation also features a strong female character--Arkady Darell. The first three books in the series were written in the early 1950s, and at times it shows. Asimov considered himself a feminist and created strong female characters (especially Susan Calvin in his Robot stories) but even so there are blindspots and occasional gender fail, because class? This was the fifties! The trilogy is dated in other ways--technological and social advances Asimov didn't foresee, but for all that I think this is still a fantastic read rich in ideas. Asimov returned to the Foundation Series in the 80s with Foundation's Edge and other sequels and prequels, merging aspects of the Robot series with it. But though I find those stories entertaining, I don't find them quite as thought-provoking as the first three Foundation novels. However, I do think Asimov's most amazing works can be found in his short stories, not his novels. I'd particularly recommend the anthology Room is Earth Enough if you can find it. His "The Ugly Little Boy" and ""The Dead Past" are in my opinion two of the best science fiction shorts out there.