How many sequels is too many?
I'm not talking about movies, though goodness knows there are several that have quite a few--Star Wars, Star Trek, ... But what about books? There are quite a few very successful series out there. And there are many more that have fizzled and died. What makes a successful series?
I believe one element that might make for a successful series is the ability for each book to stand on its own. Almost all series are more enjoyable if you read them in sequence, but some are written in such a way that each book truly stands on its own. Jonathon Kellerman's long-running Alex Delaware series comes to mind. Most of Robert B. Parker's Spenser books also work well individually.
But many others truly need to be read in order. Certainly, the Harry Potter novels fall in that category. The problem with a series that's designed to be read in order is you lose the reader right around book five or six. Life happens. People lose jobs, have babies, move, have family emergencies, get a new job, whatever, and lose track of the books release dates. By the time they catch up, there are too many books to read or purchase so they drift away.
I suspect Mercedes Lackey has worked out the best system. She created the world of Valdemar. And then within that world, she writes short three book arcs. She even has the freedom to write individual stand alone books or duets. The reader can pick and choose or catch up at a later date. The three book arcs are set at different time periods on the Valdemar timeline. And the Valdemar series is popular enough to keep all of them in print.
Other series lose steam--especially romantic series--when they reach what I call 'endpoints'. The main characters marry. Or they have children. Or one of them dies. If the entire series is dependent on the same main characters, readers lose interest at one of those endpoints. We love our HEAs. But for whatever reason once the h/h reaches the point of "they lived happily ever after" the reader is content to move on.
Linda Howard's MacKenzie series illustrates this point. Once Wolf and Mary iron out their problems, we're eager to move on Joe's story and then the other children. Conceivably, she could have written half a dozen more centered about the MacKenzie family. But each book stood firmly alone within the series. The glimpses we caught of the other characters were intriguing and satisfying without requiring their stories to continue. And two of the sons never had their own stories (though I continue to hope!)
Perhaps what we need is a new word to describe subsequent stand alone books placed in the same world. They really aren't sequels or series. So what are they? How do we describe them in such a way as to let the readers know they don't have to read them 'in order'? Any ideas?