Writing a series really shows your commitment to the art of writing, and to the love you have for the first book in the series, because if you feel the undeniable urge to continue that story then that book you just wrote is a powerful creation. However, you shouldn't write a series just to write a series. There has to be a reason for it; a story line that can be built upon and expanded through three or more books.
The role of the first book in a series is just like any other book. It has a beginning that captivates, a middle that informs, and an end that explodes. It introduces the characters and the story, but it also has to make readers want to read more.
The books that follow the first in a series have to grow and essentially get better. After all, no one wants to read a series where the first book is the best. What would even be the point of writing a sequel if it's not as good as, or better than, the first? Essentially, the second book has to remember everything that happened in the first one, but at the same time it also has to create a new story.
What happens in each book has to quilt together even if the plots of each book are vastly different, which is the catch! A series is a segment of stories; each is different but they are all linked together.
Whether your series is four books long or thirty, each book has to grow. Perhaps not in pages but with the overall impact: what happens, what's at stake, what the protagonist goes through, etc. Then the very last book has to tie all the books together with a tidy little bow. It also has to be more explosive than all the others. More importantly, it has to end the entire series, which is a big job.
Don't feel daunted though! Your characters will tell you how the series should end, so listen to them! If you do get stuck, take a step back to clear your mind. And don't be afraid to rewrite!
Chrys Fey created Write With Fey, a how-to blog about writing a novel. Every Tuesday there is a new post containing tips, inspiration, insight into her series, and much more. http://www.writewithfey.blogspot.com