Books

I answered rather lengthily about the reading order once upon a time when someone asked something similar. Hang on, I'll see if I can find it.

EDIT: I'm a lazy bum, and I found my reddit post in my native language. So I ran it through google translate, and it produced the following. Note that the below was a comment reply I made to someone who had started watching the TV series.

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This question is more complicated than you might think, and I'm too lazy to answer the phone. Check back in a couple of hours when I have a keyboard.

It is worth noting that the series takes quite a lot of freedom in relation to the books, but I think that is necessary in order not to end up having to find new actors for each season, as the books take place over several thousand years.

Hokay, reading order...

I read them in the order in which they were released:

 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)

..and it worked fine for me. But since you've started watching the series, I'd recommend changing it up a bit, as the series starts a bit before the 1951 book and enters the prequel books (the two "last"), so I'd recommend you start with the two . So then we land on the order:

 Prelude to Foundation (1988)

 Forward the Foundation (1993)

 Foundation (1951)

 Foundation and Empire (1952)

 Second Foundation (1953)

 Foundation's Edge (1982)

 Foundation and Earth (1986)

BUT, it so happens that there are also some other books that Asimov wrote, which were actually originally unrelated to Foundation. But after the Foundation books from the 80s and 90s came out, these became quite relevant after all, as the two series are connected by a number of common characters and background history:

 The Caves of Steel (1954)

 The Naked Sun (1957)

 Mirror Image (1972) <--- Short story that is not as important as the others.

 The Robots of Dawn (1983)

 Robots and Empire (1985)

...These work very much as the basis for worldbuilding, and might be a good idea to start with ahead of the Foundation series. Here you will find out what the thing about Demerzel is. Regardless, it's a good read.

In conclusion, I would like to say that Asimov usually wrote short stories, so the list of books is not as scary and long as it might seem. With that said, I would recommend checking out a lot of the other stuff he wrote. My favorite is "The end of Eternity" (1955), as well as a good number of the short stories he wrote.