Why do so many long-running series work so well? ‹ CrimeReads

I don't believe most authors set out to write a series that lengthens from three or four to eight or more. We start out with what we know, then readers demand more so we happily oblige. Of course, we all hope the work we've put into planning out that series will be rewarded with avid readers, but what elements cause readers to care enough to keep reading? I decided to take a look at my favorites and why I love them.