a. Foundation - FOUNDATION begins a new chapter in the story of man's future. As the Old Empire crumbles into barbarism throughout the million worlds of the galaxy, Hari Seldon and his band of psychologists must create a new entity, the Foundation-dedicated to art, science, and technology-as the beginning of a new empire.
b. Foundation and Empire - FOUNDATION AND EMPIRE describes the mighty struggle for power amid the chaos of the stars in which man stands at the threshold of a new enlightened life which could easily be destroyed by the old forces of barbarism.
c. Second Foundation - SECOND FOUNDATION follows the Seldon Plan after the First Empire's defeat and describes its greatest threat-a dangerous mutant strain gone wild, which produces a mind capable of bending men's wills, directing their thoughts, reshaping their desires, and destroying the universe.
It was such a fascinating series, the scope, the political intrigue, the characters. I remember loving it. Now since then, Asimov did write other books in this series, those I haven't read.... yet. In 1981, he was persuaded to write a 4th book, Foundation's Edge (I may have read this, but I can't remember). He wrote another sequel, Foundation and Earth (1986) and then two prequels, Prelude to Foundation (1988) and Forward to Foundation (published after his death in 1993).
After the Foundation trilogy, I got quite hooked on his Robot books;
a. The Caves of Steel (1954)
b. The Naked Sun (1957)
c. The Robots of Dawn (1983)
d. Robots and Empire (1985)
These books were a combination of Sci-Fi and mystery. They elaborated on Asimov's 3 basic laws of robotics;
These books were also excellent mysteries featuring detective Elijah Baley. In The Caves of Steel, he works with robot investigator Daneel.
Over the years, I've enjoyed many other Asimov stories. In 1966, he wrote The Fantastic Voyage, which was turned into a movie with Raquel Welch, amongst others. It tells the story of a team which must be miniaturized and injected into the blood stream of a scientist who is dying and their efforts to defeat the cause. In 1987, he wrote The Fantastic Voyage II, which while not a sequel to Fantastic Voyage, was an attempt by Asimov to improve on some of the flaws he felt were in the first story.
In 1950, he published I, Robot, which was a collection of previous short stories that he put in one book. As the title indicates, they all dealt with robotic issues, each story told by Dr. Susan Calvin to a reporter and each dealing with a moral, ethical issue involving robots and challenges to the laws of robotics. The book contained 9 stories and in introduction. Each was fascinating as I recall.
Now it's possible I may have read more of Asimov's Sci-Fi contributions than I realize as he has been prolific. Of late, I've begun to explore his mystery contributions, the Tales of the Black Widowers, but those don't belong in this discussion. (Well worth checking out, mind you). The complete list of his works can be found at this link.
I hope you enjoy exploring his works. Have a great week!