If a story is popular, readers want more. Sometimes pressure from fans forces a writer to produce a sequel, but often a book is written with a series in mind. Some series are finite (they tell an overarching story that concludes at some point), while others are more open-ended.
But both types of series can be extended. Even if the main series story-arc is completed, there can be loose threads that lead to more complications. Isaac Asimov extended his Foundation trilogy, as did Frank Herbert with Dune (initially planned as a trilogy, then extended into six books, and I believe Herbert had plans for a seventh).
The Dune example is more interesting, though. Since his death, others have taken over writing in the Dune universe, taking Herbert’s ideas and expanding in all kinds of directions.
I read one of those ‘new’ books recently—Brian Herbert & Kevin J Anderson’s The Winds Of Dune. It doesn’t add to the end of the series but fills in (some of) the events between the first and second books.
It’s another way of extending a series—take events alluded to in the main series, and expand them into their own stories. I think of these series-extensions as ‘side-stories’—not vital for those wanting to follow the main series story-arc, but fascinating for fans of the series.
And it’s a technique used more and more often. Writers can produce short stories or novellas as ‘bonuses’, either for publication on their own or in anthologies. Stephenie Meyer did this with The Short Second Life Of Bree Tanner, a novella following one of the minor characters from her Twilight series. Stephen Donaldson resurrected a discarded chapter from The Illearth War (his second Thomas Covenant novel) as the (long) short story Gilden Fire. In the Star Wars universe we have Rogue One and Solo, along with the ever-increasing TV shows.
Fans of a series always want more. They want to explore the lives of their favourite characters. They’re intrigued by back-story. They want to know what happens next, and also what happened before.
I’m currently reading Chris Fox’s Void Wraith Origins series. It’s a self-contained trilogy, but it’s also a prequel to his six-book Void Wraith series. He’s done something similar with his Magitech books, writing a six-book Magitech Legacy series to complement the original.
He’s also extended his Deathless series (originally a trilogy), and has tied that to the Magitech world.
Multiple series, all based in the same world, all connected.
The idea is to have a ‘flagship series’, one that is the focus and the most popular, and then build more series around that. Star Wars is a prime example, with the multitude of book series, the animated series such as Clone Wars, and the newer additions like The Mandalorian and the upcoming Obi-Wan Kenobi.
Star Trek has spawned a number of ‘spin-offs’ (Voyager, The Next Generation and so on), as has NCIS (with NCIS: Los Angeles, NCIS: New Orleans, and more reportedly in production). The Harry Potter series has spawned Fantastic Beasts.
In books, romance writers have long exploited this idea (because romance writers are always ahead of the curve). They might have one series following three brothers, then have another series focused on a group of friends, but both series will take place in the same small town. Friends of the love interests in one book or series will spin off into their own stories and series.
It’s great for readers—they get that holy grail of something fresh (in a new story) with something familiar (a world they know). And it’s a great way to write, because it takes away the need to world-build afresh with each book or series.
It’s something I’m doing at the moment. I wrote a trilogy a few years ago called Shadows, a sci-fi with horror leanings, and even though I liked the way the three books work together (they feel complete), there were still open loops, and more I wanted to explore. So I’m now working on a new-but-connected series, ShadowTech, following on from the end of Shadows. I’ve pulled back on the horror elements, and this new series has more of a sci-fi adventure feel, with a larger scope, faster pace, more action. It’s different, but still connected.
Hopefully, what readers want — novelty alongside familiarity.