There are different ways to do series. Some suspense and mystery writers have central characters, detectives or investigators, who solve the crime of the particular novel. Some romance novelists base a series on a particular town and focus each novel on a different couple within the unique and attractive setting. When it comes to science fiction and fantasy, usually the series is bound together by a created world or universe. Sometimes the same characters appear in each novel though not always.
It's up to the writer to find the right technique to bring readers back for more. I read a lot of suspense and mystery. Jonathan Kellerman brings me back for more and more in his Alex Delaware series because the protagonists are so interesting and each novel has complex psychological mysteries. I can seldom figure out the twists when they come. Science fiction writer Alex J. Cavanaugh used a complex, interesting character as a continuing thread in his Casa... Series and drew readers back because they wanted to know the rest of the story about that central character.
Sometimes romance novelists introduce a secondary character in the first book of a series, making the character interesting enough for the reader to care about them. But then the writer sets the secondary's story aside, hoping the reader will want to know what happened to him or her and thus buy the next book. I used this technique in my most recent romance series. In The Marine's Queen, Vin, a secondary character suffers a tragic loss and goes off on his own. The book ends with his fate unknown. Much to my delight, a few readers of The Marine's Queen emailed me and asked me if I intended to write Vin's story.
In fantasy series and other suspense novels, though there might be a pause in the action at the end of each novel, the writer needs to leave unanswered questions or unresolved dangers. With each book, the stakes have to grow in importance and the road to solve the problems or the mystery has to grow steeper. The world can't be saved or the evil completely defeated until the last book in the series.
In my most recent fantasy series, The Morbunda Saga, the war is just getting started in First Dragon, the kickoff novel. Disasters and losses pile up as I introduce the reader to the Morbunda fantasy world. The complexities of the characters are revealed over the course of the novel. Hopefully readers will want to know what happens next.
What techniques work best for keeping you involved in a series? What tantalizing hooks do you use to keep a series successful? What series of books have you really enjoyed as a reader?