I guess I'm just not into series of books, it's too bad because people who are into them seem to act like there's a lot of great material available for those who are into it. My wife's into that, but as far as I can remember presently the only series I even only sort of read was the Narnia books and Harry Potter, and the latter was only because I was reading them to my progeny (I read the Narnia ones to them too).
Seems like it would be an awesome career to write a successful book series, on the other hand. Lots of famous writers figured out how to be formulaic without appearing formulaic, at least superficially enough for us to suspend judgment----oh yeah that reminds me, I did read a lot of Agatha Christie, those were fun. She sort of wrote series, Poirot.