Science fiction and fantasy lend themselves to series writing. The world or worlds can be as big and complicated as the author wishes, so it can easily take three or more books, or movies, to tell the whole story. George Lucas's vision for Star Wars was immense. In order to get it made he had to focus on one tiny part of the whole story. That success led to the next two movies that seemed to end the story of Luke Skywalker. But the success and the demand for more allowed the planning and filming of the three prequels (episodes 1-3). And now with Episode VII, the last three of Lucas's original vision will be produced. How closely it still matches his idea from the beginning I do not know.
Other fantasy series I've enjoyed are David Eddings' The Belgariad and The Malloreon. He's written others, but I haven't read them...yet. Piers Anthony has a delightful series called the Xanth novels. It is pretty much magic gone wild. But if you hate puns, it might not be for you.
Stephen R. Donaldson's The Chronicles of Thomas Covenant is a series of ten novels. It was originally a trilogy, then he added three, and then four more. I've read the first six, but not the last four. There are limits. Terry Brooks has the Shannara series. The first, Sword of Shannara, shows distinct influence (some say copying) of Tolkien's work. The later books, though, not so much. I've read the first trilogy and some of the second...I think. It's been a long time. I do recall one thing. There's an entire sequence in one of them that I have used in my own writing as an example of something to never ever do. The sequence involves a man joining the quest who is a sort of entertainer. He is very good at imitating sounds, especially animal sounds. Later on the group finds itself in a city made entirely of stone and the only inhabitants are giant rats. What these rats find to eat in a city of stone with no other inhabitants is never mentioned. But the group is threatened by these giant hungry rats and the entertainer manages to buy them enough time to escape by imitating a giant cat...but sacrifices his life in the process. The group continues on. The entire illogical sequence of the entertainer and the city and the rats could have been completely eliminated and not affected the story at all. It was just several thousand words stuck in there with no lasting significance. The editor/publisher had no business leaving it in. Just my opinion. I wish I could remember which volume that was in.
One of the first and best science fiction series is E.E. Smith's Lensman series. This was originally published as six individual novels with some character carry-over and the continuation of one main plot shown at two different levels. The series is now available in two hardback volumes. Considering that the settings visit other galaxies in our celestial neighborhood, it may be the biggest sf series there is in terms of size of the fictional world. Another series written only a few years later is Isaac Asimov's Foundation trilogy. This story also has a huge board—all of the Milky Way galaxy. Decades later Asimov added two sequels to the trilogy, and then later two prequels. He also tied the series in with his "Robot Series" that featured his Three Laws of Robotics—a staple reference in science fiction and psychology and cybertronics still today. One of the unique aspects of Asimov's science fiction is that there are no extra-terrestrial intelligences. Only human beings have peopled the galaxy. In Smith's Lensman series, the galaxy is teeming with all kinds of oddly-shaped intelligences, some far stranger than anything appearing in any of the Star Wars movies. Robert Heinlein's books often feature extra-terrestrial intelligences, and Arthur C. Clarke has some in his stories also.
Other science fiction series include the Tek novels by William Shatner (yes, that William Shatner), The Lazarus Long series and the World As Myth series by Heinlein. The "series" aspect of these books is simply because they all deal with the same themes and characters, but each book is a stand-alone novel. The one exception might be The Cat Who Walks Through Walls, since it ends in a bit of a cliff-hanger. But there is no promise of a follow-up at the end, either.
And there are two series that are sort of a fantasy-in-science-fiction. One is Anne McCaffrey's Dragonriders of Pern a series that totals 22 novels and two volumes of short stories. The first novel of the series, Dragonflight, is a compilation of two novellas. The first won McCaffrey a Hugo award and the second won a Nebula Award. She was the first woman to win those awards.
The second series is John Norman's Gor books. This series takes place on "the counter-Earth"—a planet like Earth that is always on the opposite side of the sun and so we can never know it's there. This was written before we had the interplanetary satellites that would detect such a thing. This is male fantasy fiction involving life forms not of earth and many different cultures derived from old earth cultures. Swords, arrows, spears, etc. are the weapons available. This is a series you will probably either love or hate. I like Norman's writing style, generally. The series has 34 novels. I enjoyed the first six, some more than others, and kept track of them pretty much through number 12. After that I felt his commentary on society and other philosophical matters overshadowed the action that I liked reading and I abandoned the series.
That's all for today. There is something exciting currently in the works regarding my own writing, but I'm going to hold off until it's a sure thing (as much as a sure thing can be in publishing) to officially announce anything.
Thanks again for reading.