I was talking today with a professor about narrative endings and the complications that arise when a book ends. When you close that last page, the illusion ends: whatever reality was in that world ends when the book comes to its literal last page.
Some books end with a too recognizably conventional ending. Whatever the places they took you during the plot, they still end on that same note. I also went over this with a different professor last semester when I was working on a project on performances of Much Ado About Nothing in the nineteenth century. In that play, the plot goes into some strange places and asks some complex questions, but it still ends with the "happy marriage." The nineteenth-century story I remember comparing this to was The Woman in White, which does similar things with gender and marriage before returning to a traditional, marriage ending.
The question I am coming off of all of this with is, what do I want from an ending? What do you want from an ending?
Sometimes the "modern" (I put that in quotes because I think other time periods had their own version of this phenomenon, as with serialized books, for instance) trend of writing ongoing sequels as the mark of a successful book bothers me. It bothers me because I feel that, sometimes, it makes the writers forget what was important about this story to begin with. It takes away the focus and thus the quality.
Each story that is told has a reason to be told. If it was told in the first book, then you have to have a new, additional reason in order to merit a sequel. In other words, a book ends when we have heard what we need to hear about that story.
This is why it normally isn't a good thing to hear every detail about a character's life, from birth to death. Some stories tell all of this; sometimes it is a good thing and sometimes it just doesn't work. If we learn everything about a character's life, every part of his life must be important to some overall point. So if in To Kill a Mockingbird, we only hear about this brief period of time in Scout's childhood it is because the story is about what happened around Scout during that particular time. A story about Scout at age 60 would be a completely different story. So it would be ridiculous to write a sequel to the book where Scout is getting ready to go to college: that isn't what is important about this story.
So a book ends naturally, on its own. The ending shouldn't feel incongruous: even if it is a jarring ending, it must have its precedents in the rest of the text. It should make some sort of sense, even if it is unexpected or whatever else. It's satisfactory in the way that it complements all the preceding pages.
(I think I have mixed up the concepts of where, timeline-wise, a book ends and where it ends thematically. Sorry about that.)