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What does it take to keep a long-running series fresh? How do you keep readers from losing interest after a few books?
As far as series go, my first thought was the Pendergast series by Douglas Preston and Lincoln Child. It's kept me hooked for more than twenty years. I'm trying to analyze what it is that has kept me coming back. Off the top of my head:
The plot is the main thing. What's the new mystery Pendergast will solve? How will he do it? What new places will I visit, what new things will I learn, what legends/mysteries/conspiracies will I learn about? And I get more out of it because I actually care about the cast of characters.
I'm fascinated by the protagonist, and the authors have done a good job of never revealing too much about him, his history, his family of origin... Thus, there are always new revelations about him along the way, new layers of the onion to peel back. Pendergast himself is like a mystery that's been slowly unraveling over the decades.
I never worry that the protag will die (even though I feel the same reading a stand-alone) but the authors aren't above offing an important secondary character along the way, even one who has played an important role in many previous stories.
The authors do a good job of carrying subplots from book to book, or across several books, making sure the important characters change as a result of previous books, that their personal lives move forward...
It's fun to see a familiar face from several books back pop up again and play an important role.
The authors also occasionally go back and pluck little bits out of previous books and make them important to the current story.
I will say that I've lost interest in lengthy series that tell what amounts to a single story stretched out over several volumes. I guess it's the Robert Jordan/George RR Martin effect. With fantasy, I currently prefer authors like Mark Lawrence, who write trilogies and actually put out books on a regular basis.
Those are my thoughts on keeping a series fresh. What are yours?