One of the most enduring and beloved SF tales is Isaac Asimov's Foundation Saga - not just the first 3 books [originally serialised in the 1940s], nor the two sequels [Foundation's Edge (1982), Foundation and Earth (1986)] which explicitly link up Asimov's Empire stories with his Spacer stories and bring the tale up to its mid-point. Not just his early and pre-Empire novels [Stars Like Dust, Currents of Space, Pebble in the Sky] which set the scene and fill in a rather mysterious history - Foundation begins in 12,069 GE as the Empire is falling. Hence lots of story to tell. Not just the Spacer/Robot novels which link the "Spacers" of his 1949 story "Mother Earth" with his most famous creations, the Law-abiding Robots, the pinnacle of which are the humaniform robots - robots that are essentially human, just made better.
Peripheral to these he also has his Lucky Starr juvenile series, set in the Spacer Galaxy [perhaps], seen from an oppressed Earth's viewpoint. Also there is his time-travel tale The End of Eternity which involves an infinity of alternative futures manipulated by "Eternity", an organisation that controls History to keep humans safe, but denys us the Galactic Empire and loses the Galaxy to other intelligences as a result.

Even the Spacers have a prehistory, before interstellar travel, as inhabitants of artificial space colonies, as depicted in Nemesis.

And finally his Prequels to Foundation, which tell Hari Seldon's story, and themselves inspired [commissioned] sequels by three of SF's best, Greg Benford, Greg Bear and David Brin.

Here's a rough timeline by another Asimov fan...

http://www.4literature.net/story/2003/1/21/94638/5941

... the Foundation Saga covered so much of the author's life [1942 - 1992] that it is naturally inconsistent in its internal chronologising. Included in the above 'line are several novels commissioned by Asimov to expand on the Spacer/Robot stories - something purists might object to. I don't know. As a Foundation fan the subtle and not-so-subtle aging of the tales adds to their charm. Asimov was sufficiently aware of technical change to imagine advanced display and computing technology even in the late 40s, but he sets their implementation millennia hence - oh well, can't always get it right. His later explanation of the "hyperspace Jump" as a tachyon conversion/reconversion seems quaint in an era of [theoretically] manipulable wormholes, Brane-worlds and Alcubierre warp drives..