That doesn't automatically mean one should only write series. If you are a young writer, it might be wiser to experiment in numerous genres until you hit your stride and wait to invest time in creating series once you've found the sweet spot --stories that you like to write and readers like to read.
Some genres lend themselves to particular types of series more than others. Romances rarely if ever span several books with the same characters. Romance arcs are usually constrained by reader expectations of a happy ending, not a cliffhanger. Romance series tend to be joined by locale or by theme, spanning numerous discrete pairings whose stories might or might not overlap.
Mystery series tend to follow the same sleuth, but move from case to case, again, eschewing the cliffhanger model. Readers expect a mystery to be resolved by book's end--to be a stand-alone product. The sleuth might develop over the series, or he or she might be a more steady force and the appeal is the new intellectual puzzle rather than character development.
It's in adventure, science fiction, and fantasy (and their subgenres, like dystopian) where cliffhanger endings and incomplete arcs are more the norm. But look at series like Harry Potter, and you'll find that each book has a complete, contained arc, while each book also contributes to and moves forward a larger, whole-series arc. Whether you could create such a series by building on a stand-alone is debatable, however. Rowling's work clearly was heavily planned and structured to give equal weight to each volume's arc as well as the series arc. So I'd think twice about attempting to take your stand-alone fantasy and expect to have a series arc pop out without having been planned it, with seeds planted that have yet to come to fruition.
With those genre-trope caveats out of the way, I'd like to suggest some ways to build series when you've written stand-alone books.
|Some of McCaffrey's Pern series (via Amazon.com)|
Frank Herbert's Dune series follows several different characters through a universe he creates in which space travel is made possible through an altered-mind state caused by a rare drug, Spice, found on the desert planet Arrakis. Whoever controls the Spice controls the universe.
Anne McCaffrey's Pern series take place on the planet Pern, where human colonists genetically modified lizards into dragons in order to fight a sky-borne menace called Thread. Books cover everything from the first colonization to generations of dragonriders over centuries, and include other professions in the planet's guild system during its "middle ages," such as healers (Nerilka's Story) and bards (Dragonsinger, Dragonsong, Dragondrums).
If you've spent considerable time and effort building a unique setting, consider how you might use the setting for other stories, focused on other characters and/or other segments of society. It doesn't necessarily need to be a fantastical or otherworld setting either. A New Adult author might work with a particular invented college campus with unique majors or unique campus features. A cozy mystery writer might set all of the stories in the same region with different amateur sleuths. A literary fiction writer might follow several generations who live in the same oddball town.
Imagine how the unique setting might change over time because of the events in your stand-alone. Consider picking up with your main character's children or grandchildren, or with a secondary or even tertiary character you wished you could have developed more in your first book.
L.M. Montgomery's Anne series contains seven books, five that focus on Anne Shirley, and two with her children, Rainbow Valley and Rilla of Ingleside. This series follows Anne from childhood, when she is adopted by the Cuthbert siblings, into her teen years, college, early career, marriage and motherhood, moving to several locales in Canada. Once Anne is fairly settled and no longer having madcap adventures, her kids carry on.
Perhaps the sidekick character in your first book would like his or her own story. Or perhaps you'd like to carry forward what happens next from the love interest's point of view, as Gayle Foreman did with both If I Stay / Where She Went and Just One Day / Just One Year. Perhaps you'd like to experiment with changing genres without switching brands, so spin off a younger or older character and write his or her story in your existing world, but write it as middle grade, or young adult or adult.
If you feel like no characters are begging to have their own story, and you want to try a new setting, consider building a thematic series of stand-alones. The books might have the same kind of content--all coming-of-age, all awkward romances, all entrepreneurs struggling with start up businesses. Or they might have complementary themes, like Melody Carlson does with her True Colors series, each a faith-based story about a teen struggling with a particular social problem, like peer pressure, substance abuse, jealousy, heartbreak, abuse, depression.
Even if your stand-alone book tied up several loose ends, there might be some that you chose to leave to the reader's imagination, merely hint at, or simply chose to not address for fear the denouement would feel unrealistically tidy. That's the case with my second book, Almost There. It picks up a year and a half after my first book, which deals with my main character losing her dad. But while I gesture toward Dani and her mother heading toward a better relationship, I leave somewhat open ended what that might look like in the future. And her mother's family, Dani learns, have a history of dysfunction that's only briefly examined in Never Gone.
Think about the How to Train Your Dragon films. While Hiccup and his father have largely reconciled at the end of the first film, it remains to be seen how their relationship will change as Hiccup matures from teenager to man. Plus, the first film hints at the hole left by the loss of Hiccup's mother--a loss shrouded in mystery. That mystery comes to the fore in the sequel.
Unfinished business stories work only if you love your character enough to stick with them into their future. What parts of your initial novel weren't tidily tied up? Conversely, which tidily tied up things might, in time, fall apart? What minor characters lurking in the background want to come forward and interact with your protagonist? What aspects of your protagonist's flaws do you believe will loom large and cause conflict in the future? Build on your previous story, considering where natural consequences would lead over time.
If time has passed since your initial release, it's wise to work to make the sequel understandable as a stand-alone itself.
What are some of your favorite books series and why?