So, being a Tolkien scholar, I'm professionally interested in unfinished books. THE SILMARILLION is not the end-all and be-all of Tolkien's literary achievement, and Tolkien wd still have, and deserve, a major literary reputation even had the SILMARILLION never been published, or indeed failed to survive. But we're fortunate to have it.
Even more fortunately we have both THE SILMARILLION as editorially assembled by Christopher Tolkien (1977) as well as the many constitute parts he presented in chronological sequence between 1980 and 2018. And this enables us to witness Tolkien's struggle to find the right form and format to present his mythology, and to compare it to the similar woes of other authors immeshed in parallel difficulties.
Here's I'm thinking not of the kind of unfinished book like Dicken's THE MYSTERY OF EDWIN DROOD or Austen's SANDITON, where the author knew where he or she was going and simply had the misfortune to die before reaching the end. Pound's THE CANTOS may fit in this category: the poet certainly failed to provide the grand synthesis at the end that he'd promised at the beginning of the project, but it's impossible to tell whether this was due to a flaw in Pound's schema or failure due to his encroaching mental illness (or both).
I wd also set aside Poe's THE NARRATIVE OF A. GORDON PYM or Coleridge's KUBLA KHAN, where I take the 'incompleteness' as a narrative guise assumed by the author.
Closer is Hawthorne's DOCTOR GRIMSHAWE'S SECRET, where the author flailed around, uncertain of characterization or plot, having a setting and a whiff of an idea he can never come to grips with, no matter how many times he returns to the beginning and tries again (i.e. he knew Dr. Grimshaw had a secret but had no idea what it was). By contrast Twain's THE MYSTERIOUS STRANGER shows an author who knows what he wants to say but can't find the right presentation, struggling between three radically different versions that were editorially assembled after his death, rather like Christopher's SILMARILLION. To give another example, I'm currently reading David Lindsay's THE WITCH, which its author reluctantly put aside, having written himself into a tangle he cd not get out of, that of attempting to present the ineffable in words. It's hard not to feel for an author who desperately wants to finish a work but just can't find the way.
And then there's the work which is not so much unfinished as unbegun: a sort of phantom text that exists mainly in the mind of the writer, with v. little if any of it actually set down on paper (or extant in electronic files). The most notorious example is probably Truman Capote's ANSWERED PRAYERS, excerpts from which he described in detail, consistently, over a long period, yet precious little was found among his papers at his death. To pick another example recently in the news, Harlan Ellison's THE LAST DANGEROUS VISIONS anthology was frequently announced as 'forthcoming' for the last forty-five years of the editor's life; promises of a posthumous edition are being met with a certain skepticism.
In contrast to works obstructed by writer's block, some works remained unfinished because the author had too much to say and cdn't apply an internal editor, like the main character in the film WONDER BOYS. I understand this was the case with Thomas Wolfe, whose novels were extracted from a wordy matrix by his editor, Max Perkins, but have not looked into that case myself. Certainly Ellison's JUNETEENTH (aka THREE DAYS BEFORE THE SHOOTING) and Foster Wallace's THE PALE KING seems to fit this pattern.
In the end I'd say we're lucky: to borrow Tolkien's analogy not only do we have the soup that is the 1977 SILMARILLION but Christopher Tolkien gave us guided tours of his kitchen for a behind-the-scenes look at how it was all put together (the History of Middle-earth et al).
And we can be grateful that JRRT didn't meet the fate of the writer's-blocked author in Clark Ashton Smith's unsettling story "The Nemesis of the Unfinished".
--current reading: Scott Berg's LINDBERGH
P.S. On a personal note, I shd add that the eventual release of the Beach Boy's famous unrecorded album SMILE made my friend Franklin Chestnut very happy.