If you're looking for a large book, here is a rather VERY large book.
https://www.kobo.com/us/en/ebook/the...-of-the-fallen
It's some 10,083 pages long!
Actually the number of pages changes. In the link I provided, it actually states 11,927 pages, the description on amazon says 10,083 pages. And in a couple of epub's from the library one is 8,344 pages and the other is 8,244 pages. I think it's just the font sizes selected for the books that are causing the differences and not (hopefully) missing pages.
Regardless, that's one massive book.