One of the fun things about writing a series with the same characters and setting is the ability to weave certain subplots throughout. With a stand-alone book, the author must meticulously wrap up all loose details.
But with a series, certain elements can continue from book to book, even unresolved issues.
Jesse Stone series. We learn in the first book that Jesse is still in love with his ex-wife, and though she has remarried, she obviously doesn't want to let Jesse go. He moves across the country but he is still tethered to Jen through the telephone. She phones, and it can change his entire mood - or drinking habits.
At the end of that first book, things don't get wrapped up with Jen. I've never heard a reader ask, "But what happened...?" It's clearly a layer that is going to continue through the series, just as people in real life often have unresolved issues and relationships.
Further on in the series, Jesse loses his job. His drinking has gotten out of hand and along with other occurrences in the book, we find that America's favorite small town police chief is out of a job. It's there that the next book begins, and we don't know whether he will be reinstated or if the new chief, the son-in-law of a city councilman, is going to prevail.
When you are reading a series, do you look for everything to be wrapped up in each book? Or do you know that some things will continue from book to book? What is your favorite series and why?