I've been thinking about series for a number of reasons lately. One is that the Scribechat folks were discussing series this past Thursday; another is that they were discussing Janice Hardy's Healing Wars series, and I've been lucky enough to be on the inside of the struggles surrounding putting that series together (struggles which ended quite victoriously, I might add). A third is that I've been encountering series issues in my own writing.
A series is when you have more than one story set in the same world. Generally this also means that the books in the series also have other things in common. Characters, or plotlines, or themes, or all of the above. They can be stand alone shorts which follow one another, or stand-alone novels which follow one another, or they can even be different parts of one massive story.
So what is absolutely essential to make readers care enough to follow a series?
1. A compelling world.
The world doesn't necessarily have to be physically large to carry a series, though a large world does leave lots of room for exploration. However, there must be something about the place that fascinates - something that keeps readers asking questions. Janice Hardy's pain-related economy definitely does this for me. I read books and books in Anne McCaffrey's Pern series, and I think what got me about that one was the idea of the link between humans and dragons that grew in response to the ongoing threat of Threadfall. The draw for me in Tolkien's books is the history of Middle Earth and its different peoples, and the complexity of their interaction. I'll also be more inclined to read more in a particular world if I feel that there are things going on beyond the borders of the page (i.e. the world does more than simply serve the plot) - so that if our heroes turned right instead of left, they might encounter something new and interesting.
2. Characters you care about.
This is absolutely essential. Some series even rest on the shoulders of a single individual. Whoever your characters are, people have to care about them and what they want, why they want it. They need to care that the good guy win and the bad guy lose, or they won't keep reading. I would even go so far as to say that it's important to give the proper attention to your minor characters. If you achieve huge success with your series, who knows how many opportunities there might be to spin off characters?
I don't believe that a series must stick with the same character set (necessarily). However, there should be a link between the characters of one book and those of another if they are to be perceived as part of the same timeline. Bilbo Baggins had a small role in The Lord of the Rings, but still he was there. I'm currently attempting the same thing with a prequel-to-trilogy that I'm putting together. The characters from the book I'm currently writing - Nekantor, Tagret, and Aloran - aren't going to have point of view material in the trilogy that follows. But they are terribly important to the precarious situation in which the society finds itself, and to which the next three point of view characters are going to make the biggest difference.
I really think those are the only things that are absolutely necessary. Other factors are more flexible.
People often talk about a slump in the second book of a series, much in the same way that they talk about a slump occurring in the middle of a novel or shorter story. I think in some sense these are manifestations of the same phenomenon, but at different levels of complexity. Certainly if the series is one integrated story, the problem is similar. The middle section has to have its own interest and momentum, even though it doesn't get to have the "splash" associated with initially discovering the world. It has to have its own compelling reasons to exist, even if it's not going to be able to solve everything. Stakes in a second book have to be higher, but not so high that you have nowhere to go with a third book. There has to be enough that readers haven't yet discovered, both about the world and the characters, to make it worth continuing.
I don't consider myself an expert at this yet, but as I learn from people like Janice, and approach my own sequels and series (at both long and short lengths), I'm finding myself wanting to analyze and write down what I feel I do know about the process. And I thought it might be useful to share those thoughts with you.
Here are some great thoughts on series revision from Janice Hardy over at The Writing Cave.