The topic of book series is one fraught with mixed opinions. There are authors who swear by them and ones who would be far more content crafting a standalone novel. And weirdly, both opinions can be equally valid.
Book series make a great deal of sense from a number of perspectives. Drawing on the same world and characters allows an author the chance to bank on and build the affection readers have for the first book. This not only increases the odds of maintaining sales with each new offering in the series, but raises the likelihood of building a loyal fanbase that will spread their love of the work to other readers.
Multiple books also allow the writer to more deeply explore the workings of the fictional universe and its denizens, further investing both him and and his readers in the ongoing story. So, not only does this build additional loyalty to the series as a “brand,” but it allows the author breathing space to tell an epic that simply can’t fit into a single novel.
Which all sounds great, right? And it is … for those with epic stories that truly warrant the length.
The major issue with any book series is bloat. The longer the series, the more likely a misstep becomes. It’s incredibly difficult for any author to keep the same world and characters interesting after so many outings. One mediocre book too many and fans may become disillusioned and turn away, accusing the author of continuing purely for money (sometimes, rightly so). The writer also runs the risk of burning out on the material, especially if he never had a solid overall arc in mind when he started the venture. And even if the quality of a series is more or less consistent, the longer it runs, the greater the demand made on readers, particularly new ones. A series consisting of more than three books comprises a time and financial commitment that people with a passing interest may not be willing to offer, and they could even be intimidated by the expanse of material in more extreme instances.
So, like most things, book series are a fine route for the right author–and the right story. But a writer needs to be careful not to overestimate the legs of the material. Because as satisfying as a well-told trilogy can be (and on a personal note, I believe there are very few stories that require more length than that), sometimes 300 pages are more than enough to do the job well.