Was Dune meant to be a trilogy, which Children of Dune finished, before Frank Herbert decided to write more? The first three books are often grouped together, and have a similar time-frame and subject matter, so I wonder.
If there is no information on this matter, letting me know of that (whether as an answer or in the comments) would also be helpful.
Dune was originally published as two shorter serialised novels called Dune World and The Prophet of Dune, and I know of no evidence to suggest that Herbert was already planning any other sequels at that time. So Dune was originally a two-part novel rather than a trilogy, and both parts are contained in the novel now published as Dune.
According to an article from The Verge:
Herbert began work on what he envisioned as a trilogy of novels, but eventually packaged the three together into one massive tome that was eventually serialized in Analog Science Fact and Fiction in the first half of 1965. Dozens of publishers passed on publishing the novel until Chilton (a publisher of thick automotive repair manuals) bought the rights and released the first edition in December 1965. Ace Books produced a cheaper paperback edition in 1966.
Even as Dune gained considerable popularity in the 1960s, the effort of writing and publishing the novel had worn its creator out. It wasn’t until 1968 that Herbert began to write a sequel called Dune Messiah, that continued the story of protagonist Paul Atreides, and his new role as a messiah and emperor.
He did not plan the first three novels (Dune, Messiah, and Children) from the beginning. He planned for the material covered in the first novel, Dune, to be a trilogy. Instead, he included it all in one novel. It does not appear he initially planned out a story stretching over Messiah and Children.
If I remember right from one of the (fairly) recent anniversary editions of Dune (don't have it in front of me at the moment, think it was the 50th), the forward written by Frank Herbert says that he did tons and tons of research into economics, agriculture, religion, and more for writing Dune, and he did so much preparation for the novel that by the time he finished writing Dune he had the material and plans for writing the first three novels. I think the second was mostly finished when Dune was published, and the third was at least all planned out.
I don't remember it saying how the stories were originally published in magazines. He may not have started with the intent of writing a trilogy, but when he was done he knew it would be a trilogy and more.
I may try to update this answer with excerpts from the forward when I have a chance to check it.
My understanding is that Dune was originally published by Frank Herbert serialized in a magazine. His original intention was not write a trilogy. That happened purely due to pressure from his publisher.
