Re: Dune
doublelayer,
My favourite author, C J Cherryh has written about this. The problem with trilogies. You have a big idea. You need to make it huge, as you're covering a new setting, new characters, maybe even multiple alien cultures. Here's my 800 page book. Your publisher looks at you in horror. They can charge £12 for a standard paperback. Maybe stretch it to £15 if it's a really fat one - and that no longer fits in their standard shipping systems. OK, we'll split it into two 400 pages novels. Job done! But to split it you've not got to add a "false ending" to the first episode. So you probably need to re-write and re-order it a bit - add a bit of extra build up and stuff to one of your existing bits of plot - this gives a reasonably satisfying ending. Now book two needs a re-introduction of the characters and setting, you need to know who's where, doing what and why. You may even need to add to the plot a bit, to make up for the changes you made to give you an ending... Oh dear. Page count is now up to 1,000, instead of two very fat books - you end up with 3.
Next thing, those three were good - hey! Why don't you write a series! You might not win any new fans, but nerds like to collect things - so once you've sold the buggers the first three - they're doomed to buy everything you put out, form a sense of completeness.
Universes / settings, on the other hand, seem to work much better. e.g. Niven's Known Space series, where things connect and the author probably has a timeline and history/backstory, but the individual books are often unrelated. They seem to end up much less as cash-cows.
Cherryh has done both. Late in her career she's fallen into a horrible loop of producing more Foreigner books - the original trilogy of which are great, I'd say the second and third trilogies are also very good - but the quality has been dropping for ages. I guess the publisher keeps asking for more, as she was talking about self-publishing other stuff at one point. Much of her other, and best, stuff is in the Alliance-Union universe. Many are tightly knit together, telling the story of Earth going to space and then having a war with its own colonies. The biggest series in that is a couple of trilogies, but it's mostly stand-alone books. And she's described that as like history, don't read them in order, except the linked ones, read them as you get to them - and then piece things together. Downbelow Station and Cyteen are the ones that won the Hugo award.
I guess the thing about series is to find the ones where the author wrote something really good, and only then added more. Say Orson Scott Card with Speaker for the Dead and Ender's Game - and even Ender's Game was apparently written up from a short story because he wanted to do the sequel and it needed the context. But they're both amazing, in totally different ways. The 3rd, Xenocide completely lost me - the whole idea seemed too silly, and then I've not read any of the rest. But the first two are genuinely great books. But often the first had to be good, or they wouldn't have commissioned a series, and then you just need to rein in your natural desire for completeness, and stop reading.