Am I writing a series, or am I writing one really long book? Don't panic, I know my answer, but it's an interesting question for those of us who have continuing characters. My detective will appear in at least three books (maybe more...) and all the books will follow one another chronologically. But it's not one big story, it's a series.
The argument could be made, for instance, with the Harry Potter series that the entire seven books is really one big story. J.K. Rowling had one enormous plot that took seven years to tell and every book is another step closer to the final denouement. Practically, however, no publisher would ever publish anything that big. And let us never forget the financial goodies that came along with seven different books and having the resultant rewards spread over many years.
Writing a series comes with its own issues. An author must pay heed to a continuing character's history. If he broke his leg in the last book, does he now have an ache in the leg when it rains? If she said she was going to Spain at the end of the last book, do you make mention of it at the beginning of the new one? Or does the new one occur in Spain? Habits have to continue, (or be broken) but the character must continue to evolve. If the new book takes place 5 years later than the last one, then did your character age 5 years as well? Do you explain what's happened in those years?
Continuing characters are wonderful gifts for both writers and readers. They give readers a chance to meet up again with familiar faces as well as getting introduced to new ones. Writers don't have to bid farewell to a character they love to write. I can only imagine the agony that Dame Agatha went through when she decided to kill off Hercule Poirot and I'm sure Ms. Rowling felt the same pain when she finished telling Harry's story. Continuing characters become the writer's familiar friend and letting them go must be a wrench to the heart.
If you have a continuing character in your writing, think about it for a minute. Are you writing a series, or could it be one big book? What is your attitude as a reader? Love continuing characters? Or do you want to hit them with a shovel?