Let's talk a little bit about series novels; books that span an arc of multiple titles, from three or four to a score or more. As somebody who has plied those pulp lanes myself a time or five, I am admittedly biased in their favor, and I thought I might address in passing the why of that.
It's true that a good stand-alone novel can be a treasure that needs neither sequel nor prequel. The tale is told, it is complete, and adding more would gild the lily. Lonesome Dove, say. Moby Dick. Said everything that needed to be said, and more isn't necessary.
Even though, like Dune, sometimes more is necessary to keep bread on the writer's table.
And sometimes fans demand it. I would love to have seen a sequel to Zelazny's Lord of Light. Actually, I'd love to be the guy to write it ...
Other books can be made richer by further adventures. Lord of the Rings would probably not have been so epic jammed into a single volume. Hard to imagine Asimov's Foundation Trilogy as a single book. Travis McGee needed the room. So did Mike Hammer; George R.R. Martin has what? Eight million pages? done in the Fire and Ice saga, and the monsters haven't even come over the wall yet ...
Our culture loves expanded stories. Look at movie sequels or series television.
It seems almost axiomatic that the first or second movie in a popular franchise is the best of the bunch. After that, bullets fly every which way. Aliens? The first one was spooky, the second one better, III and IV? Sucked. And yet, I went, even though I was sorry ...
Find a drama you love, a soap opera, sitcom, and you'll tune in week after week. If it's a cop or spy or science fiction show, the suspension of disbelief is pretty big to keep any kind of tension going, because unless an actor gets pissy and fired (or actually dies), they aren't going to kill off the star of a network show and everybody knows it. You have to pretend there is real jeopardy. If it gets too scary, you can always comfort yourself knowing they won't kill her. Leastways on network, non-cable stuff.
Cable can take 'em out left, right, and center, and does. Sometimes they decide to do so at the end of a run, ala Saving Grace, but they do kill major characters in ways that would make NBC, CBC, ABC, and even Fox cry out and have seizures.
And where are the movies for Deadwood, I want to know?
Beckett gets shot in the heart at the end of the season? You can bet the farm and all the red leg chickens she'll be back next year. If you have a big ensemble cast? Yeah, you can knock off red shirts or supporting characters hither and yon. But: Jim Rockford was clunked on the head enough so he must have felt like a tent peg at a traveling circus, and I never once worried that he would die from that, or being shot, or being thrown off a cliff. He was the show.
There have been exceptions to the rule, but they few and far between, especially on the non-cable shows.
(George Martin kills 'em like flies in his fantasy novel series. I mean, nobody is safe, including your favorites. If you haven't read them, trust me here, some of the folks you love on the HBO series die in the books. No need to say "die horribly" because that's what George does.)
In a book series, once you are hooked, you tend to look forward to the next one, even if the quality declines. And the quality almost always does decline after a certain number, which varies from writer to writer. I was told by an editor once that an SF or fantasy series was usually good for ten or twelve books, and after that, people tended to stop buying them. Some of my favorite mystery writers in a series couldn't keep me interested by the tenth book. They got into a pattern, the books flew on auto-pilot, and at some point, I shrugged and stopped going on that flight.
Others, even though they were getting tired and repeating themselves, kept collecting my money because I was so attached to the characters I couldn't let them go. John D. MacDonald was tiring of Travis and Meyer, the villains generic psychopaths, but as long as he wrote them, I bought them. Of course, he had a level of quality below which he didn't fall, and at his worst, was much better than many other writers are at their best.
Sue Grafton lost me halfway through the alphabet, mostly because her detective kept leaving her gun home and doing stupid shit.
After the Burke novels, I tried a couple more of Vachss books and gave up. I'd kept turning up for Burke and his extended family, but without that, the newer ones didn't do it for me, and even that series had gone down. The first five or six were lean and mean, the last few got really preachy. (We tend to do that, writers, and I include myself here. I am aware that I had fans who read my stuff for a while, then elected to stop. I understand that.)
I read all of Parker's Spencer series, even though they were after a while clones. I liked Jesse Stone better, he was a more interesting character, but Spencer and Pearl and Hawk and Susan and the assorted back-up were like having a beer with old friends and telling stories.
The point of this, I suppose, is that stories take what they take to tell them properly, and some can be short and sweet, and other longer and drawn out. Tapas or a sit-down dinner are both satisfying, in the right circumstances. And many of us are into to the super-long form, probably because we think that if a little is good, then a lot is ever so much better ...