It started with television series. A show that holds your attention by continuing a story from one season to the next, often with a “cliff hanger” at the end of each season.
This has become commonplace in fictional books as well. I’ve accepted this phenomena for digital streaming/network television content, but it just doesn’t suit me in books. Sure there is that rare book that so captures my attention that I can’t wait for the follow-up book, but so often the sequel doesn’t match up to the first that it becomes frustrating.
Personally I think the reason for the book series concept is largely greed or laziness. The book publishers like the idea because they feel they have a committed and surefire following. The book authors like the simplicity of not having to invent a whole new set of characters and story line.
Throughout history there have been books with one, maybe 2, follow-up books that seem to work just fine, like Jack London’s Call of the Wild and White Fang. Even there, most consider these books to be companion books. At any rate, I prefer a single story that you can savor and contemplate as you finish the last page and close the book. For me it’s “one and done”!