I guess it depends on what you mean by “incomplete”. AFAICT, Billy Budd isn’t exactly an “incomplete” novel in the sense of having a complete beginning part but no ending part, like Austen’s Sanditon or The Watsons.
Rather, it was more of a Tolkienesque chaos of revised and expanded versions that the various later editors had to sort out for publication. Everything that happens in every published version of Billy Budd, AFAIK, is something that Melville actually wrote. It’s not as though another novelist took over the job of determining the events of the narrative.
Finally figured out why I thought there was a second half to this.
Stuart Palmer was Rice’s good friend. After she sank into alcoholism, he helped her by writing a series of stories combining his Hildegarde Withers with her John J. Malone. Putting two authors’ detectives together was highly unusual for the day and the series was a big enough hit that Palmer kept writing them after Rice’s death. (She contributed maybe an outline or two and a few pages to the originals.) I personally love the weird pairing of opposite personalities.
But the funnier connection is that when Palmer died, he left an unfinished Withers novel and that too got completed, by Fletcher Flora, Hildegarde Withers Makes the Scene (1969). Unfortunately, it’s awful.
Rice was a beloved character, despite her many flaws. After her death Larry Harris tried to continue the Malone books with The Pickled Poodles (1960), one of the earliest examples of another author openly writing a character. It’s not at all good, though. There was also 1957’s But the Doctor Died, which was published under Rice’s name but not one word is in her style. Hint: it’s dreadful.
The result is that both Rice and Palmer are almost completely forgotten today despite being first rank stars until the early 1950s.
Continuing on the Czech theme, one of the most significant classical Czech novels is The Good Soldier Švejk. It was completed by a journalist, but the version I bought and read was the incomplete version.
And it sort of works as an incomplete story because it’s about a soldier in the first world war, almost in journal format; the writer, Jaroslav Hašek, did survive the war, but died young, before he could complete his planned work. The character is one you could imagine surviving well and living long, or not surviving at all, like so many soldiers and even civilians didn’t. Having no ending, in that context, actually fits better than having someone else complete it.
It’s very good. You have to read it bearing in mind that (if you buy the version I did, at least), it barely saw an editor. But some of the imagery has held with me over the 25 years since I read it.
The other unfinished novel that I have read the “finished” version of is The Salmon of Doubt, by Douglas Adams. Adams wasn’t the sort of person who planned his novels that much, so Eoin Colfer was more or less going on what notes there were, what Adams had said to his wife and friends, his other stories, and his general style. But it was, more or less, fanfic by a decent writer.
And the actual published version had to be cobbled together from drafts that sometimes conflicted (which they are also honest about). It just about hangs together as a novel that is nice to have if you love Douglas Adams, and see where he saw Arthur going. Otherwise, not really.
That one may not be a good example, but Heinlein finished John W. Cambell’s ‘Sixth Column’, but he substantially rewrote it all to remove as many of the racist overtones as he could.
Neil had the first part. He didn’t know what to do with it, so he sent it out to some people. Pterry is the one who responded and the rest is History. So…maybe.
Collaborations happen in a thousand ways. However, very few of those include someone else finishing what was started without the originator being involved.
Gwendy’s Button Box, by Richard Chizmar and Stephen King.
King wrote the beginning, got stuck and busy with other commitments, and got Chizmar to look at it and complete the rest, with feedback from King. Chizmar wrote a sequel (with Forward by King), and another “button box” collaboration novel from them is coming out next year to conclude the “Gwendy Trilogy”.