And then one considers the series, which is a work of art published in parts. Sometimes. You get works like The Lord of the Rings which is one book that had to be chunked into three because of the limits of book binding. And you get works with the same characters or merely setting that really can be read in any order. And you get everything in between. Like Harry Potter, which had threads carrying from book to book, but the first five were also stand-alone works that had closure in themselves. I think the commonest is some multi-volume arcs, but the book stands on its own.
From the commercial POV, the requirements are obvious: you want a series where a reader can pick up book number 5 and not go "Huh?" and put it back on the bookstore shelves -- especially since, alas, he often won't be able to move his hand to number 1 and start there. With any backstory from the older books neatly supplied as needed.
Aesthetically, the question gets more complex. Even in series "Of simple plots and actions the episodic are the worst. I call a plot episodic when there is neither probability nor necessity in the sequence of episodes." You have more leeway because the books are works in themselves, but not infinite.
The best series, in my experience, are unified works of art. And to unify them, they need arcs. But then, you need backstory to understand the later books, and supplying backstory neatly is not easy when a good chunk of your audience knows it already. It gets particularly ugly when you end up with an omnibus edition. I (fairly) recently read an omnibus of a duology. The first book even ended with a cliff-hanger. But in the first chapter of the second book, there was some fairly awkward info-dumping for people who hadn't read the first.
Allusions to earlier works are easier, if they aren't integral; an off-hand comment can sound just like the sort of unwritten backstory you slither in to make the world more real.
More difficult, however, is set up. If a series is not to be episodic, a book needs to have its present events not only explained, in least in part, by its past, but foreshadowing the future. OTOH, this can interfere with the present action. The book itself may lack closure. Which can work (like The Lord of the Rings) but even in a novel, or a short story, things that only set up other stuff are not wise. Especially not long passages of them. And you can't get much longer than a book. . . .
It can be weak even in a story. I was recently reading a "story" where some soldiers, on a mission to atone for a failure, decide to give it up, murder those in their number who resist, and set out -- with a lone soldier fleeing to bring back news of the treachery. And I was thinking, this had better be a teaser for a novel. 'cause the soldiers didn't fell conflicted about giving it up, the murders were in fact murders not fights, so they didn't even have a external conflict -- not with the one who fled, either; they deduced it was him, hailed him, objected that he didn't stand forth to be murdered, and let him go. It read like some connecting events in a novel, and would not have been scintillating even in context in such a novel, but it might have worked; as it was, it was not a story. "A whole is that which has a beginning, a middle, and an end. A beginning is that which does not itself follow anything by causal necessity, but after which something naturally is or comes to be. An end, on the contrary, is that which itself naturally follows some other thing, either by necessity, or as a rule, but has nothing following it. A middle is that which follows something as some other thing follows it. A well constructed plot, therefore, must neither begin nor end at haphazard, but conform to these principles." -- and this one was all middle.
(Not to mention a story in the same anthology, which was a complete story, but I, having read the earlier works, was picking out the arcs with an eagle eye and some of the events might have been frustrating if I didn't see how they could lead to something new.)