Another difficult form of ending is an ending where you know there’s going to be a sequel. This can be difficult because there are questions to answer at the end of book one, questions that will be answered through the series, and the things the reader might not even realise are questions in the first book.
Finding the right balance for these can be difficult but first you need to decide what the important answers are for the book you’re writing at that moment are. For example, if we got back to the saving the world plot, it might be: What happened to the villain? Did the protagonists survive? Do I need to drop a hook (plot hint) for the next book at the end of this one?
The multi-book/film/episode will-they-won’t-they arc immediately comes to mind (perhaps because I’ve been reading a book with that plot). If we know the protagonists aren’t getting together at the end of book one but we know we want the reader to read the next book to find out if they do then we need to answer the question: Why? Why don’t these two characters get together at the end of the novel? Is there hope that this might change? Why should the reader want them to? All these breadcrumbs need to be dropped through the novel, particularly something that makes the reader want these characters to get together, we’ll go with consensus and call it ‘chemistry’.
So we’ve built up the character’s chemistry through the book, we’ve got the reader involved in their relationship, they want these two together, they’re perfect for each other so why the heck aren’t they together at the end? Once again we need the breadcrumbs: Is there another potential love interest? Is there some circumstance that pulls them apart, their jobs for instance? Is there a secret one is hiding from the other?
My point is that simply because there’s going to be a sequel we can’t say ‘they’re not together because there’s going to be another book, to be continued’. We have to make the reader want to continue for a logical reason rather than frustrate them a ‘because they didn’t’. Buffy and Angel from Buffy the Vampire Slayer were complicated to begin with by the idea of relationship between a slayer and a vampire. Lois and Clark in Superman were complicated by Clark Kent’s dual identity as Superman. Sookie Stackhouse’s relationships in True Blood/Southern Vampire Mysteries were complicated by choice of partners. These situations offer questions such as: Can they overcome the fact they’re meant to be enemies? Will Clark tell Lois about his superhero identity? Can Sookie wade through the emotional entanglements to pick one or none? (It hasn’t escaped my notice that it’s usually picking one of two possible partners)
The point is that these are valid questions formed from complex situations rather than conspicuously keeping characters apart for the sake of more books. It might detract from the ‘craft’ or the ‘art’ to point out that on the most basic level our readers have invested time and money in our books and have expectations. Not to give a valid reason to keep two characters apart is both a frustrating ending and gives a sense that we’re out for their money, not to tell a good story. When writing a series we have to bear this idea in mind that not only do we need to give a reader a good story but we need to justify them buying the next book. While it is the same with any books we write when they’re sequential there is a sense of justifying the longer series rather than stretching what would work as a shorter story over multiple books.
As such an ending for a series book needs to work both as an ending and as a beginning that allows the unanswered questions to draw the reader to the next book.