How to end a book in a series
Obviously, we want to tie up loose ends, but with a series, we set the stage for the next adventure.
This can be done in a multitude of ways. A short scene, an epilogue, or even an incoming message can be a setup the next book.
Ending with a short scene
So, our MC returns home after saving the people on Mars from extinction, only to find a stranger in their living room, who tells them what they did to solve the Mars problem caused a bigger problem for the IO colony on Jupiter’s moon.
This sets up the next book, where the MC will have to do something to help the people on Jupiter.
A short scene like this doesn’t need a full chapter or epilogue, but it can be stretched into one if you want.
Ending with an epilogue
Epilogues can be used like the above example, or they can show what the next adventure could be.
Instead of having someone tell the MC what the new problem is, there is a longer scene where the reader is shown the damage caused on Io. Volcanoes are erupting with increasing intensity and the colonists are being killed due to these eruptions. They are scrambling to build fire resistant buildings and shelters, creating better oxygen systems to filter out the ash, but they are struggling to survive.
With this type of setup, the MC isn’t aware there is a problem, but the reader knows they will be called upon to solve this problem.
Ending with a fast teaser
One of the quickest ways to hint at the next journey is to have the MC get a message.
As soon as they walk into their house, their device sounds. They look down to see who it is and find the name of someone they met from Io during their time on Mars.
That’s it. The reader will assume the person from Io isn’t calling to chat, but to ask them for help.
There are probably infinite ways of setting up the next book in a series. These are just a few examples to get your mind roiling with ideas.
Any way you set it up, it’s a giant cliffhanger.
It leaves your readers wanting to read the next book, which, of course, is your goal!