The WordPress/JetPack writing prompt for today is:
What’s the longest book you’ve ever read?

I’d be happier if the prompt was “What’s the longest story you’ve ever read” instead of “book.” The definition of book can get fuzzy.
Technically, The Lord of the Rings was written as five books, originally published as three books. And yet, it’s one complete story (and eventually published as one, once printing presses could accommodate a book spine of that thickness).
If it is one book, as I consider it is, and if you include the appendices as part of the book, it’s over 575,000 words (still over 480,000 if you exclude the appendices).

Leo Tolstoy’s War and Peace is over 587,000 words, which puts it in the category of “epic” according to this chart. But I haven’t read that one. And I don’t know if it was first published in its entirety or in smaller chunks.
If we consider C. S. Lewis’ The Chronicles of Narnia as one story (released as seven books), the word count is 344,000. And Philip Pullman’s response, His Dark Materials is slightly longer at 384,000, told over three books.
Frank Herbert’s Dune was ’only’ 188,000 epic words, but his complete story (told over 6 books) is 854,000 words, which is longer than the King James version of the Bible (783,000 words, excluding commentary, 824,000 with commentary).
But a story isn’t about the number of words needed to tell it. It’s about the completeness of the story, the way it engages you, and if it leaves you thinking afterwards.
At least that’s true for me.