Climb into my time machine, or the Delorean I don't have, and type in the date Summer 1985. "Sweet Valley High" series. I LOVED THEM! I was in middle and high school when I read these. I used my allowance on the newest book and waited anxiously for it to come out. When I grew up, I wanted twins. Jessica and Elizabeth were now my favorite girl names. Then either I matured, or the storyline got old (or both) and I stopped reading them. And, I don't have twins...
Currently: I love the Stephanie Plum series by Janet Evanovich. The books before #12 were "Laugh aloud on the plane and have people stare at you funny." I looked forward to seeing how Stephanie's car was going to blow up this time, what trouble and shenanigans Grandma was going to pull...then the books went on. And on. And Stephanie hasn't made a decision between two men, she still has her hamster from book #1 and even though she has upgraded from a pager to a cell phone there hasn't been any movement towards a finale in the life of The Plums. I feel about this series as I feel about the tv show The Simpsons. It's time. I know it's not a chronological book, but it's time for her to make a decision and move on.
"Outlander" by Diana Gabaldon is one of my favorite books. I watch the STARZ tv series. I read the first four over and over again. Then, when the characters' children grew up and had children of their own and their stories came into play, I gave it one more book...and then I gave up. I got tired of being bounced back and forth from country to country, time period to time period. Lord John seems to be one of Diana's favorite character's and he begins to trickle in and out. He even has his own series which soldified the fact to me the "Outlander" series is done and it was time to move on.
My series "Lane Changes" will have two books published, the third is written and the fourth and fifth (final one with the main character) are done but not edited. For fun, I began writing from Lane's oldest's POV moving forward and it's fun to write. Will it ever get published? I don't know. I just know writing it brings me enjoyment; I'm not ready for Lane's saga to end. So, I can see why Diana did it...and it does keep me out of trouble. But I'm not a world famous author.
And these are just my opinions. How about you? Have you come across a series of books that you think should just end because the author has exhausted the storyline?