Do you want each book to stand on its own, or are you trying to build a body of work with connections between each book?
Both. Sort of. Warden's Reign is the first novel of the Essence of Ohr quadrilogy, so there's no pretending my story opens and closes at book one. Instead, I'm working to weave a greater arc throughout the series while maintaining satisfaction in each part.
What is so satisfying about a standalone book? Well, for me, I know the investment of my time going into reading it, that's one. And the other great thing you can assume about a book like this is that the story will end with a pretty tied ribbon at the end. Most the questions are answered, and the reader is, if not pleased with the end, has some sort of closure with it. The bummer for me when I read a standalone is that it only takes up a week of entertainment before I have to start my search again for another good book.
So what about series? From duologies to stories with dozens of sequels, stories with multiple installments hold more appealing entertainment value in my eyes. Like the first book? Great here's a dozen more to fill your year of free time. It's an easier decision for me.
Why not have both? Every series should be written in a way that a reader can pick up a book, any from the sequential line up, and fall immediately into that world, unconfused and hooked. A good author weaves in the important plot points of the previous novels while maintaining the pace in the current on. The next ingredient? A well-structured story arc. Every instalment should poise their own questions, conflict, and goals, then end with a solution.
But there's a difference with story arcs: book level vs. series level. You need both. The struggles the characters face within each book should be resolved within the specific installment, yet also build upon the greater storyline or the overarching arc of the series. It comes down to minor and major conflicts and solutions. Since most of the population knows Harry Potter, I'll use it as an example. Destroying Lord Voldemort is the ultimate goal. He's the major overarching antagonist. But he isn't the sole bad guy in every book. Others like Sirius and Umbridge serve as the smaller role of the 'in story' antagonist. Each of these books deals with the smaller bad guys and wrap up in a way where we are given closure while maintaining Voldemort as the ultimate goal--he's the connection throughout the series.
The two ideas combined is what makes story magic. It's what I strive to do in my own writing. Guess we'll see
how I fair once the last book releases. :) Here's to hoping!