A reader asked me to explain something I mentioned to Terlee in comments to this post over at my writing blog while discussing series length. Since it concerns the Disenchanted & Co. novels and has an impact on world-building in general, I thought I'd write a longer post about it here.
First, the two comments involved:
Terlee: Three seems too short (okay, too short for me to get my Dredmore fix). I like the idea of five, though really, you're the only one who knows how much is left to tell...
Lynn Viehl: As a series writer you always have to keep things a bit fluid. I admit, five books would give me enough room to tell the story I envision, but three would insure I'd finish it. So you can see my quandry. :)
As a traditionally-published writer I never know in advance how many novels I will publish in any given universe. I can plan all I want, and even sell all the rights I want, but it's the publisher that decides that issue. Sales are always the primary determining factor for any traditionally-published series; if the books sell well the publisher will be inclined to invest in more. If they don't, the publisher won't offer another contract or may even cancel books under contract, and the series ends. One of the great advantages of self-publishing is that authors can now continue series that were dropped by publishers, but writers on my side of the fence are almost always waiting for the ax to fall.
This is why keeping a series story fluid is so important, and by fluid I mean kept open to the possibility of more books but also written so as to serve as the final novel. Balancing both can be difficult, but I find that building and focusing on a standalone conflict in each book while gradually building and advancing a connected but entirely different series conflict in every book is the best approach.
In the Toriana books Kit has puzzles to solve in every story, and these mysteries are the standalone conflicts. While she goes about her business, she also deals with her developing relationships with Dredmore and Doyle, as these are the catalysts for the series conflict. Since I like writing about relationships as they develop over a long period of time, which I consider to be a lot more interesting than the one-book HEA variety, this approach is one of my trademarks as a writer. In fact you can find similar series conflicts in my StarDoc and Darkyn novels with Cherijo and Reever and Alexandra and Michael respectively.
You don't have to use a relationship as your series conflict to keep things fluid, but I believe you need some strong and enduring story element that can be a part of each novel and thus tie your books together. This can be something like a quest, a setting, an event of long duration, or even a character group (for example, lots of romance writers go with stories of family groups for trilogies or short series, in which all the series protags are brothers, sisters, siblings or closely related.) We've all seen books that tout the now-classic "brotherhood" character groupings as catalysts for series conflicts, too.
Whatever you use to tie your series together, it should not be a disposable or easily-resolved conflict; by design it should be able to endure. Some writers invest a series conflict with lots of complexity when building it to be introduced in the first book, but I go the other way and stick to very simple and very strong opposing forces -- yin and yang would be one way to put it. These conflicts inspire me to no end, and always serve as amazing foundations on which I can build.
You should also have an endgame in mind for your series conflict, so that if you do know in advance what will be the last book you can wrap things up accordingly. Often as a traditionally-published author you don't, but now that we have digital self-publishing there are more options for series writers, which I consider one of the huge advantages of going indie -- you can write as many stories in a series as you want. :)