There were actually two series prequels, Prelude to... (1988) and 1993's Forward the Foundation
(and three more, if you've read Benford, Bear and Brin's 'Second Foundation' trilogy: I haven't yet).
Then you have the classic trilogy (which started out as collections of stories originally written between 1942 and 1950, as said), i.e. Foundation, Foundation and Empire and Second Foundation (1951-53). The latter was my first introduction to sci-fi literature (ayup: started from the last), pretty darn near 30 years ago. I was smitten.
Finally there are the sequels, Foundation's Edge from 1982 and Foundation and Earth from 1986.
Before Prelude there are the robot short stories, the four robot novels (the first, The Caves of Steel from 1954, is excellent, and one of the top five among my list of favourites by Asimov) and the so-called Empire trilogy (better described as the Radioactive Earth series).
Re the TV series, I'm watching and hopefully going to enjoy it as it is, but not really as an adaptation of the original work, which I still consider unfilmable, not unlike The Simarillion is - as in: you'd really need to pad it with non-original dialogue and characters in order to make it work as an AV, and in so doing, turn it into something else.
Which is pretty much what they're doing (in both cases - a Lord of the Rings TV series, based on the story of how the titular character gained...the title, originally told in The Silmarillion, coming up next year).