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:

1. Foundation (1951)
2. Foundation and Empire (1952)
3. Second Foundation (1953)
4. Foundation's Edge (1982)
5. Foundation and Earth (1986)
6. Prelude to Foundation (1988)
7. 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.

1. Mother Earth (1949)
2. I, Robot (1950)
3. The Caves of Steel (1954)
4. The Naked Sun (1957)
5. Mirror Image (1972)
6. Bicentennial Man (1976)
7. Robots of Dawn (1983)
8. 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)