I posted this as an answer about series writing over on sfnovelists.com, but I wanted to post it here as well, so it would be, among other things, easier for me to find.
I currently have two worlds in progress, and they’re very structurally different. The first, the DAW books I write as Michelle West, aren’t really self-contained, and they’re certainly not short. I started those in 1994, and have continued to write them, and for me? They’re not finished yet. They don’t have an ending. I had envisioned a number of ending arcs, emotionally, for a large cast of characters– but I am only approaching the end of one of many character arcs in the newest set of books; the end of the six book series, The Sun Sword brought one of the key characters to the midpoint of the arc I’d envisioned for her before I ever mentioned her on the page.
Because I’m not finished, I don’t have that sense of restless frustration one has when there’s nothing new to experience or explore. Because I know where it’s going (if not always how it’s going to get there), what’s happening in the present of the book resonates with the ending I see in the distance, and it moves me. It probably bores a lot of people, though.
The novels I write for Luna books, as Michelle Sagara, I envisioned as more episodic (in the Buffy sense). And as I reach the end of each one of those books, I’m finished. I’m done with the story; I’ve said what I had to say. There’s more to say – there is an overarching season-arc, to borrow the Buffy analogy again – but I want to start saying the new stuff now. I want to explore other corners of the world, other races, other crimes. The world is both broader and less detailed, and that was intentional – because if it wasn’t, I thought I would lose steam, lose the sense of the immediate and the new that comes with exploring new terrain.
And when I reach the end of the longer arc, I want to be done. I will have run out of things to say at that point. And while I know people will prefer one book over the others, I don’t want them to feel that I’m phoning in my lines; I don’t want them to feel that I should have stopped, and just kept spinning in place.
Michelle 
bonnie h 
Gretchen 
Carrie 
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Michelle,
Keep up the great work. I have really enjoyed all your books, especially the Sun Sword series. After having read The Hidden City, I went back and reread the Sun Sword series. Of all the ‘series writers’I follow, I have found that each of your books equal or surpass themselves, in my eyes at least.
Thanks again for your wonderful books.
Sue