Some readers prefer each book in a trilogy to be somewhat stand alone. That’s because they do not necessarily want to read the other books or they just want to grab one book randomly without having to read the others.
Sometimes readers think that the plot could have been handled in just one book, so the following plots feel stretched thin to them.
In other words, if you do not offer a satisfying resolution at the end of the book, readers could be frustrated.
EXAMPLE 1: In Time Snatchers a fun series by Richard Ungar, the hero (Caleb) has to face his tyrannical boss and another thief, Frank, who snatches his assignments. Both villains gang up on Caleb, but want different things. At the end of the book, neither problem or villain is really tackled, and there is no satisfying resolution. Thankfully, the story is still fun to read. But instead of the traditional structure, the author decided to give a good resolution to a secondary plot. Caleb saves one of the kids his boss decided to snatch and ends up living in his family. My son enjoyed the book immensely, but when the last chapter was read, he kept on asking me to read the end of the book. He could not believe this was the end.
EXAMPLE 2: Some stories leave you wanting for more because they have a satisfying resolution, but you sense it’s not the end of the hero’s ordeal. Twilight ends with Bella at the hospital. She has escaped a group of vampires, found a new family among Edwards’s friends and secured Edward’s love. Belonging, winning Edward’s resistance and staying alive were the main problems in the story in book 1. At the end, all these conflicts are solved, but we understand that all is not over yet. We understand that Bella’s life is not going to be easy and there is still a lot to conquer.
There's something to be said about leaving you wanting more and more in Science-fiction and fantasy novels. Some stories can't be told in one volume. Some stories get better and better as they are being extended. Sometimes a trilogy leads to a better world-building and more epic plot lines.
For contemporary fiction, a stand-alone book might work better because there is not as much world building.
Signs that your story is complete:
- In trilogies, if your characters regress, go through the same emotional upheavals or relearn lessons, that means a follow up book is unnecessary. You just keep on writing and re-writing the same story.
- If you characters stay the same and do not evolve or change, you do not need a follow up. Readers like to see the characters change and build from what they’ve learned.
Harry Potter is a series of stand-alone novels. That does not mean that the characters do not change over time. The relationship with Lord Voldemort changes and the characters grow up, get into different skirmishes, fall in love, develop different concerns and interests depending on their age (belonging, popularity, self-esteem, solidarity, reaching out to other schools, developing formulas, exploring deeper parts of their worlds, taking part in contests, saving animals, etc.)
Even the secondary characters evolve like Snape dwindling between good and evil.
The truth is a number of readers feel disappointed by the sequels. Other cannot read enough sequels and feel like they want to stay a little longer with the characters. These characters are unique and non-stereotypical. They are disappointed when the characters change too much after too many books. They feel that the hero they fell in love with is not there anymore. Others feel like they have to finish a trilogy because they hate not finishing a story. Others are tired of trilogies altogether. They feel like authors are holding back information in order to have enough material to use in the sequels and that fells a bit artificial or they feel like there is not enough materials for book 2 and the story drags.
Whatever the case, an author will gauge the popularity of a novel before deciding about sequels, but many will draft a rough plan of the future books just in case, to give a perspective. Being prepared is a good idea, but spending months on writing a sequel that will never take off is probably a waste of an author precious time.
Erin Bowman. "Lessons Learned While Writing a Trilogy."
Daniel Calvisi. TRILOGY BUILDING Part I: Story Mapping The Star Wars and Halo Trilogies.