Prologues are a common topic about writer and agent blogs. You can probably find a bazillion agent blog posts on prologues, what they want to see, what they don't want to see, ect. A the baseline, really, is that if you want to write a prologue, it should a) Have a purpose in the plot b) Not just be a cleverly disguised Chapter One c) be outside the realm of the characters that we will know at the beginning of the story.
Simple, right? Well, epilogues have similar guidelines.
Prologues and epilogues are not just the first and last chapter with fancy names to make us appear fancy. Like every other literary tool, it has to aid the story, and if it doesn't, it should be cut.
How do you use epilogues? I think I could flip this blog post on its head and use "prologues" in the place of "epilogues" because I believe the point is the same. There are two kinds of epilogues, what I will call the Chapter Thirty-Nine and Out of the Box.
What is a Chapter Thirty-Nine epilogue? It's an epilogue that shouldn't be there. It's an epilogue that begins after the climax and ties everything together. This shouldn't be called an epilogue, as it's the natural denouement for the book. There is nothing outside our realm of what we know of the world or the characters. The Chapter Thirty-Nine epilogue can also take the form of a "bow tie ending" where the author can go into detail about the character's life after the conflict is finished, and essentially gives them a "Happily ever after." JK Rowling did this at the end of Deathly Hallows, and I feel it really cheapens the book.
The point of an epilogue is to give us a GLIMPSE. That's right, ladies and gentlemen, just a glimpse. If we tell our readers everything that happens after the climax, how Billy became a carpenter and how Jane really did become a showgirl but Sally eventually married Tom and had kids. WE DON'T NEED TO KNOW THAT. Seriously, we don't. Ignore the fact that Chapter Thirty-Nine and "bow tie" epilogues can be found in published books. That doesn't make them right. Unanswered questions will leave your readers thinking about your story long after the book is finished. Did Billy ever make it out of that fire? Will Sally leave town after college? Will Jane ever fulfill her dreams?
Giving an "out of the box" epilogue can give readers a glimpse of the future and still leave them asking questions.
I've used the phrase "Out of the realm of understanding" when referring to prologues and epilogues. What I mean is that something in the world or the characters changes dramatically from the main bulk of the story and whatever time the epilogue takes place in.
This can mean, we get a glimpse of the villain (if this isn't a common occurrence in your book) we get a glimpse of the MC six months, two years, ten years later, perhaps we glimpse a new character from years before or years later. We get a small look outside the confines of your main character, their primary conflict, and their worldview.
What makes a really good book is staying power. You want your readers to think about your book long after they've put it down. You want them to keep thinking about it, you want it to keep them up at night, you want them to tell their friends about your book. To have that staying power, you cannot give away everything.
Show the ending, but don't tell them what happens next. Let the reader figure out what happens after THE END. After all, by the time the reader gets to the epilogue, it's not your book anymore. It's theirs.