Writing a book series is a little more complicated than writing a single novel. While the novel ends in its entirety, the book series has to continue and develop to a larger plot, one that challenges the writer and audience at a larger scale. The challenge therefore becomes how to keep it relevant and interesting at the same time.
Outline the storyline before you begin the series
A book series takes time and you might not remember everything concerning the storyline as time progresses. For this reason, it is wise that you outline the storyline by identifying the central conflict and the main character and designing a progress pattern. Gradually, you can effect changes to the storyline as you see fit but keep it as close to the original as possible.
Introduce the main characters but hold out on their backgrounds
To keep the book series captivating, you should introduce your main characters as early as possible, most especially in the first book. However, you should hold out on their backgrounds and introduce these gradually as the book series progresses. This ties each individual book in the series together while also thickening the plot.
Add more characters and eliminate others as it moves along
Adding a few characters in each book keeps the series alive as does taking out some of them. Find ways to keep the characters relevant to the story. You can also choose to take out characters and add them back in or find replacements for them. It is important to note however, that each character has to stay long enough to affect the main plot. If a character's actions are not effectively in aid of the plot, then that character should not be included.
Develop a central plot for each book
You should have a main plot, which ties each book together in the series. However, to develop this plot, make sure that each of the books in your series have a plot of their own. This gives each book individuality but also keeps the main plot alive. It is important that the book's plot does not overcome the main plot of hijack it midway, unless that was the intention of the series from the very beginning.
Before you embark on your journey to writing a book series, you should understand clearly, the differences between a book series and a novel. This will give you the know-how of keeping each book within the series but with its own independent story. Remember that for a series, there are two ways to cultivate them. They can be standalone's using the same characters, like in detective novels, or they can be a continuation of the previous story.