Wow. A tough question if ever there was one. Just how long should a book series last? Some authors love trilogies. Others subscribe to the seven-book saga made popular by J.K. Rowling. Some authors have no preset number in mind; if another idea for a tale manifests, they write it. Others are working toward a specific goal, such as Sue Grafton, the author of the popular Kinsey Malone mysteries, who is working her way through the alphabet. In fact in a review of Grafton’s T is for Trespass, USA today praised her work and wondered,  “What does it take to write twenty novels about the same character and manage to create a fresh, genre-bending novel every time?”

And that is the heart of the debate. Is an author cranking out the same story over and over or creating an original tale for each new book? Some authors get around the fresh/stale problem by constantly creating new characters. Piers Anthony and his wonderful Xanth series is a good example. His characters age and new ones emerge. His 36th Xanth novel will be published this December.

The dilemma that authors face is readers often love the characters in the series they read. They want to read more stories, find out how the next adventure will unfold. They want to see how the lives of the characters progress, learn their ultimate destinies. (Have you seen the meme that made its way around Facebook this past summer? It showed a kneeling man and the caption said, “I pray for world peace, the safety of kittens, and that J.K. Rowling’s new book is really about Harry Potter, the Grown-up Years.”)

Some readers love formula tales, books that follow a well-known, well-loved pattern that gives them the assurance of knowing how the the story will unfold and even what the ending will bring. It’s safe and comfortable, like a good friend. Others want the thrill of not knowing, the adventure of discovering something new, the excitement of wondering, “what is going to happen this time?” Does a long-lived series need to provide endless twists and surprises or does it survive by keeping to the formula? A little of both, we suspect.

Some authors are under contract to write one or more titles in their series per year. Is it hard to be creative under such deadlines? We don’t know the answer to that one since we are not under such constraints, but we have seen once wonderful series deteriorate into obvious have-to-crank-out-another-book tales. This is sad, both for the authors and the readers. Yet the money factor that drives this kind of book production is a hard fact of life.

When will the Time’s Edge series end? We honestly don’t know. We have several adventures in mind for Michael and Kate, and we do know their ultimate destinies. We even know what the final book in the series will be; the last chapter of that tale is written. We keep it in mind when we write and sometimes will hint at their final story. We have already written three books in this series and have three more planned. After that, who knows? If we have more stories to tell, we will. If we feel it is time to bring it to a close, we will do that, too.

That is the true dilemma of writing a book series: knowing when to make a graceful bow and exit.