I'm currently reading David and Leigh Eddings' Dreamers series. Eddings is an excellent teller of fantasy stories...BIG fantasy stories. If you want to investigate him a little before investing your time and/or money in his writing, I suggest you start here: http://www.fantasyliterature.com/feature/why-you-should-read-david-eddings/
I've read two or three other series by Eddings. But I haven't learned what I could have from them. Until now. The key is to write a lot. Simple? Maybe. But after reading the first three books of the four-book series, I realize that the key is to build a really big world, people it with plenty of interesting characters, and go into details on their interpersonal relationships.
I look at Witchery now, and I could make that 87,000-word book into three books of about the same length each. I could have done that, but, remember, up till now I've never thought about writing a series. I have written the stories that came to me.
Now, I'm going to write a series (at least, that's the plan). So I must approach the entire project with a different mind-set. The story must be expanded. The worlds must be large and the characters must be plentiful and—here's the key—they must have long and interesting back-stories.
Going back to Witchery for a moment, I could have included expanded back stories on Kaar and Mirra and Milla and, of course, Maalak. I could have gone through the history of the long line of Torlins that ruled Kessia from its founding. In fact, I prepared some of that while writing the book, but decided it wasn't part of the story.
BUT IT COULD HAVE BEEN!
Right this minute I'm aggravated with myself about the missed opportunity.
I'm going to derail my train of thought for a moment to reveal a truth that some of you may know, but it is likely that some do not.
That idea of the potential of Witchery as a series—the plot just as it was, with the same characters—came to me as I was writing this, and because I was writing this.
Writing actually can produce new insights.
I discovered this back in an English class in college. I'm used to learning new things about my characters while I'm writing fiction, but it's been a while since it happened while writing non-fiction. Cool, huh? Well, I think so.
Back to series writing. When I decided to do this I asked Anthony Wedgeworth how he started. He revealed that he had the concepts figured out before writing the first book, Fate of Thorik. He plotted out about thirty books and established many of the interactions and motivations between characters.
There is no way I can do that.
George R.R. Martin opined that there are two kinds of writers—the architect and the gardener. The architect plans out everything. The gardener plants a seed and watches it grow...and sees what it grows into as it develops.
Anthony is an architect. I'm a gardener. I've dug the whole and tossed in the seed. A few times a week I get an idea about what direction I want this bonsai tree to grow and I write it down. But I know I'll be surprised now and then as the story grows.