I've only written one box/thing/bundle/whateveryoucallit of the series, just off the bat, just so you know. SIREN SUICIDES grew from 1 book to 3 via 5 drafts, well, 7, if you count 1 full pass done by my editor Colleen M. Albert, and then 1 final one by me after that. Many folks asked me to share the wisdom on writing series. As such, I don't know how much wisdom I have, but I did learn a few things from writing this beast over the course of 1 year, and I think next time I decide to write series, I might be prepared to do it better. Or so I hope. Here is what I did, and you can try doing too.
Treat your series like 1 book first, like series second. I don't know how true it is in the overall landscape of writing series, and I'm sure there are well seasoned authors who have cracked this nut and will give you much more wisdom on the topic. What I learned from my experience is that by treating my book as 1 story, I was able to round it up in the end nicely, keeping 1 story arc across all 3 books, but at the same time creating 3 separate smaller arcs in each of the books alone. At first it happened unconsciously, but then in the 4th draft I stared seeing clearly the separations in the story. It was almost like certain chapters seemed to be conclusions to a much bigger story, and certain others seemed to be the beginnings. In the end, I realized something funny, and it might only apply to a trilogy, so those of you with more books planned, don't look at me funny, okay? Back in the time when I wrote screenplays, I read STORY by Robert McKee and sort of used it as my guide to write. In it he stresses the point of 3 very simple story parts, namely, beginning, middle, and end. I don't remember exactly how he calls them, so don't quote me on this, but it fits a trilogy very nicely. The beginning is book 1, middle book 2, and end book 3. This is exactly what I did, I simply expanded my book in its 3 acts, and, BAM!, ended up with 3 books.
Decide if you want to center around the character or the world. Since I'm writing fantasy, I guess we're talking fantasy here. In light of this, once you have decided on your story and on its beginning, middle, and end, you have to decide if it's about a particular character who goes in different adventures, or if it's more about the world where your story is happening. For example, compare Harry Potter and Discworld. Harry Potter goes on a new adventure in each book, where as in Discworld Pratchett sends multiple characters on their own journeys within the world he has created. I would say the majority of the series I have read center on the character going places, doing things, like in The Dark Tower, but in others the world itself plays a major role, like A Game of Thrones or Abhorsen trilogy. You can still treat those books like beginning, middle, and end, but it's rather beginning, middle, and end of your world. Or, as the story typically goes, the end, the middle, and the beginning, since at the very end your world will probably shake off some evil creature and come back to prospering, shining, and all that other good stuff. SIREN SUICIDES for me was the character driven series, but now that a few people who read 1st draft of ROSEHEAD have asked me for sequels, if I ever write them, I would have to center them around the rose garden fantasy world.
Create multiple characters and kill off multiple characters. This is not necessarily something you can do in 1 book, but if you're writing series, not only should you do it, you must do it. I didn't do enough of it in my 1st novel, still being timid and afraid as a newbie writer. I should have done more. George R. R. Martin is a master of that. The whole point of the series is to draw you deeper into the story, to make you want more, to make you experience a multitude of emotions. Killing off characters at the end of book 1, only to introduce new characters at the beginning of book 2 is what keeps the story fresh. You throw a spin on the old idea, so to say, and spin it freshly into the reader's mind. J.K. Rowling likes to do it too. In Harry Potter, if you noticed, she introduces a couple (or more) major characters in the beginning of each book, my favorite being Gilderoy Lockhart. You notice how he will get demoted, and how he gets mentioned later here and there, until one day Harry and friends discover him in St. Mungo's Hospital. It's a minor thing, but it adds richness to the narrative, makes it more real, and the more you do it, the better. My biggest problem was, I was afraid to handle too many characters in my 1st novel, so I limited myself on purpose, having only 8 characters in SIREN SUICIDES. In ROSEHEAD I braved 24! Well, 2 of them are dogs, although 1 is talking, so I suppose it counts. Anyway, ROSEHEAD is not part of the series, at least, not yet, so forget I mentioned it.
I think these are 3 major giveaways I have glimpsed through my own process. The other thing that is important is staying consistent throughout, which is very difficult, because writing series spans over a long time, and while you're writing them, you grow as a writer and your writing style changes. You have to learn to be okay with it and keep going forward anyway, without being tempted to go back and rewrite the 1st book. Instead, why don't you start new series! So, this is it. Anything I missed? Anyone? Anyone? Chime in.