There have been two long-running sci-fi series which I have enjoyed immensely in the past. Firstly, The Foundation Series by Issac Asimov.
This series spanned almost 40 years! The first book was based on a previous short story on robots – a revolutionary concept at that time. The final book detailing the origins one of the most central figure (through his influence rather than having him in each novel in the series), Hari Seldon, came out shortly before Asimov’s death in 1993.
Although this is now known as the The Foundation Series, in fact the story includes the Robots, Empire, Foundation novels (each series consisting of a number of books), but eventually tied up very ingeniously. You can get an idea of the chronology here. Note that the books listed are not necessarily in the order they were published. Apparently there is also to be a Second Foundation Trilogy, commissioned by Asimov’s estate, and written after Asimov’s death.
I can still remember, as a Year 9 student in high-school, being quite fascinated by the idea of Psycho-history when I first picked up the *Foundation*. This in a nutshell is the theory that history on a large time-scale can be effectively ‘influenced’, proposed by Seldon. In the process, his group developed techniques of manipulation based on historical forces. Naturally, the fact that later on certain groups developed mental techniques were even more amazing. Overall, the scale of his was simply amazing.
The second series which I have been fascinated with (and hopefully there will be more titles to com as the author is alive and kicking AFAIK) is the Childer Series by Gordon R. Dickson. More on that next time.