Those first three Arizona historicals all came out of existing characters-- with no advance planning. By the third, Arizona Dawn, I knew the epilogue was introducing a future heroine. The fourth, a novella, Rose's Gift, had me firmly in the camp of writing a series, with the next three in my mind before I wrote the first of the Taggert brothers (Echoes from the Past; Lands of Fire; and Bound for the Hills).
That left me with seven Arizona historicals with one more gelling around in my head by say next winter. That one has to be written, set in 1906 Arizona and California, because it's part of the families that make up the contemporary, suspense, paranormal series of an Arizona family of witches. How did that family come to use magic will be in the eighth Arizona historical.
See how series grow like topsy. They evolve and even when they are planned... sort of, it's still interesting (to me at least) how they grow into more than I originally had expected. Stories get more complex, characters become intriguing, and they show potential for just one more story.
In between writing, or at night when my back needs a break, I've been reading what amounts to a serial. I hadn't exactly planned it that way, but the author, who shall remain nameless) is a friend of other authors. I am always interested in how someone else, who does well, tells their story. It's been educational, but although this book is supposed to be funny, to me, it's not. My sense of humor is not non-existent, but I think it comes more out of natural events than what seem to be forced. I like to laugh but just not be told when I should, I guess. The heroine is such a loose cannon that I'd hate to even know her in my personal life and not that interested in knowing her through the books.
A serial is connected stories but not necessarily with any conclusions- except maybe the last one. So in the book I am reading, the author has a series of events happening, but no story arc at all. It's just walking through these events. I've looked at the reviews for this author with LOTS of reviews, hundreds. Most like her humor and style, but there were those, like me, who absolutely did not. No writer can satisfy everybody, and she satisfies an awful lot of readers; so sure she can manage without my approval.
In my own reading, I enjoy stand-alones or a series, where you can step into it anywhere along the way-- even if you'd be happiest perhaps picking the characters up where they begin. But each story has a separate love story and a conclusion-- even if there are threads running into the next stories. I've never tried writing a serial, maybe never would; but I've written several series, some lengthier than others.
I think I'll write more about the appeal of the series on Saturday as this has gotten long enough.