Whoever said brevity is the soul of wit wasn’t being paid by the word
(I don't mean to pick on Charlaine Harris, but a fan-computed timeline has calculated that the first nine books in the Southern Vampire Mysteries series span less than two years.)
Previously I've praised J.K. Rowling for planning her Harry Potter series from the beginning and sticking to the plan. I also appreciate that she spaced out the books fairly evenly, not making her fans wait a decade between books, as have some other authors.
Philip Jose Farmer's "Riverworld" series started well with two volumes back-to-back in 1971, with a third and final volume resolving the series' mysteries promised within a year. It was more like six years, and then it was more like a reboot than a sequel, which set the stage for a fourth and final volume. (Three years later, however, Farmer wrote a fifth book, but it doesn't address the central mysteries.)
Farmer originally wrote the Riverworld saga as a single volume, but when he couldn't find a publisher, he broke it up into smaller pieces. And that's OK with me. The concept ~ an idyllic garden planet where every sentient Earth human, from cavemen on up, has been resurrected for some initially unknown purpose ~ is certainly big enough for multiple books.
The other problem I have with long, drawn out series is that a writer's style often changes over time. More than a couple of years between volumes, and the parts no longer match. In the case of Riverworld, Farmer's concept of the books also changed. Rather than start over again from the beginning, he made a massive mid-course correction that irked me. Among the new characters who are introduced ~ most of whom die by the end of the book and and disappear from the series ~ are a couple of devotees of Sufism, which suddenly has a lot to do with the reasons behind the Riverworld. I wish he had completed the series as originally intended and then explored other characters and aspects of Riverworld in other volumes.
Another series I loved at first was Kage Baker's "Company" ~ a science fiction series about immortal cyborgs and the limits of time travel ~ that began with "The Garden of Iden." I thought it went offtrack when it started to resolve the mysteries it had established by introducing a non-cyborg, non-immortal who had none of the time-travel limitations imposed on the other characters. In effect, he could do anything the author wanted, and resolved all the problems as if he were God. He was even a "trinity" of sorts, working with his two clones.
While I sometimes get irritated by multi-volume sagas ~ trilogies and longer ~ I do enjoy a good series. A series gives an author more room to develop characters and explore the setting.
One of my favorite series characters is Rex Stout's Nero Wolfe, whose adventures fill 47 books written over a period of 41 years. I can't say the principal characters changed or grew much in that time ~ they barely aged, though the world around them advanced from the Prohibition era to the days of Watergate ~ but the interaction of Wolfe and his assistant/narrator Archie Goodwin make the books endlessly entertaining and re-readable to me and many other readers.
(Apparently Stout never got tired of writing them either. After a few other novels early in his career, he didn't write anything else.)
A&E tried a Nero Wolfe TV series for a couple of seasons with a repertory cast. Many of the same actors would recur every week, usually playing different characters or sometimes more than one character in the same episode. With some reservations, I enjoyed the show (it's available on DVD if you're curious), but I prefer the books. They're being reprinted again, two at a time in single volumes and as eBooks. I highly recommend them.