Many years ago, J.R.R. Tolkien submitted his much-re-written manuscript of Lord of the Rings. He described it as having been written in his heart's blood - and it showed. He'd spent years of his life working on it. It was poetic, passionate, exciting, funny, sad, and had characters you could care about. Because it was so very long, the publishers decided to divide it into three volumes, to be released separately.
Understandable, but this basic publishing decision led, eventually, to what I call The Dreaded Trilogy! (Exclamation mark intended) That is, of course, when it's limited to three volumes. A few months ago, I agreed to be put on a publisher's list for "spec fic", assuming I was going to be sent newly-published SF. So far, I have been sent Volume 13 of one series which I haven't read (well, I did start Volume 1, back when it was going to be a ten-volume saga, and gave up around Page 250) and Volume 7 of another series I haven't read.
Publishers have all gotten the multi-volume bug. Even Star Trek novels, which I used to read, even buy, when such wonderful writers as Diane Duane and Barbara Hambly were writing them, are now trilogies. There's no point in buying a remaindered volume for my school library because you can't read it without having read the others.
I remember once hearing a fantasy novelist at a con saying, "I'm starting my next trilogy next year ..." Automatically, a trilogy. Having been sent Volume 1 of the occasional saga to review, I have noticed that nothing much happens in some of these, because a story that could have been told in one book, perhaps two at most, has been stretched so that it can be sold in three or four or even five volumes. I don't blame the writers, who are only doing what they've been told, perhaps not even the publishers who are, after all, running a business. But I'm blowed if I'm going to read them.
I have always assumed that I'm in a minority - the publishers wouldn't be doing it this way if it didn't sell - but once, at a con, I suggested a panel on the subject "The Dreaded Trilogy", thinking it might attract a couple of dozen people in a small room. It filled up the convention's main room. I was on the panel because it was my idea, the rest of the members were trilogy-writers. I was embarrassed and felt sorry for my fellow panellists, when I realised how hostile the audience was. People were angry at being forced to buy multiple books to read one story.
Not to mention the irritation of being left on a cliffhanger when the author writes two volumes, say, and gets on with other works while you wait years, in vain, for the final volume. This has happened to me before. There was "The Sword and the Dream" trilogy which ended with a cliffhanger in Volume 2. If the third volume has ever hit the bookshops I have never seen it.There was a novel written by a friend of mine, which they made her cut down in length - and then let the first story go out of print and refused to publish the sequel. I believe she has ended up self-publishing it on-line, but it just isn't the same.
And then there's a certain prolific Australian novelist who produced two volumes of her trilogy over about ten years and has yet, at this writing, to come up with Volume 3, because she has been flat out working on other stuff in the mean time.
To be honest, I have rather gone off fantasy, anyway, with a few exceptions - I love urban fantasy, such as written by Charles De Lint, and humorous fantasy. I can read Terry Pratchett's series because, although it helps to have read the others, you can more or less get by starting with nearly any of his Discworld stories. There are series within the series, of course, but there are enough stand-alones that it doesn't matter so much. I like alternative universe, although you can't quite describe that as either fantasy or SF, unless, say, the aliens arrive in a world in which the South won the Civil War, or there are fairies in an Elizabethan England ruled by the Spanish. Mostly, though, since I have concluded that no one can do it as well as Tolkien, fantasy has to be damned impressive to interest me these days. And it will interest me more if it is told, simply, in one stand-alone novel, or a series of stand-alone novels.
I much prefer SF (preferably hard, but with characters you care about, such as the novels of Stephen Baxter) and space opera.
It's such a shame that some writers whose SF I used to love, such as Lois McMaster Bujold, have taken to fantasy, however good - and she does write it well. Multi-volume fantasies. Sorry, Lois. You've lost me.