The trick is, as the series went on, he dovetailed it into the Robot novels.
First, good hardcovers here:
https://www.foliosociety.com/usa/the-foundation-trilogy.html
Now, that being said, reading order, I favor publication order:
- Foundation (1951)
- Foundation and Empire (1952)
- Second Foundation (1953)
- Foundation's Edge (1982)
- Foundation and Earth (1986)
- Prelude to Foundation (1988)
- Forward the Foundation (1993)
Now...
Foundation and Earth (the last book chronologically, 6 and 7 are both prequels) hooks it into the Robot stories and serves as a capstone to BOTH the Foundation series and the Robot series.
- Mother Earth (1949)
- I, Robot (1950)
- The Caves of Steel (1954)
- The Naked Sun (1957)
- Mirror Image (1972)
- Bicentennial Man (1976)
- Robots of Dawn (1983)
- Robots and Empire (1985)
3, 4, 7, and 8 are the "Robot series proper".
Complicating matters, some of these are short stories and there have been MULTIPLE compilations of them over the years.
Bicentennial Man (1976) started as a short story and was reworked into the novel The Positronic Man in 1992, but it's the same story.
Collections are:
The Rest of the Robots (1964)
The Complete Robot (1982)
Robot Dreams (1986)
Robot Visions (1990)
Gold (1995)