Roots by Alex Haley. It was about 900 pages. Not too grueling for some, but that would a big deal for me.
A former English Professor of mine has an ongoing dare for me to read an unabridged collection of a few Charles Dickens novels that's over 1,600 pages.
I also want to commit myself to reading Anna Karenina over the summer, but we'll see.
The King James Version is (IIRC) 1300+ pages; the New Revised Standard Version, with all the notes, is a 3000+ page e-book. I studied them in school; we got all the way through Old & New Testaments at least once in each of those versions.
I've also read War and Peace, which is "only" about 1440 pages. But Tolstoy's books are great reads. I didn't really want it to end, or for Anna Karenina (a mere 880 pages) to end either.
I don't remember how long the harry potter books were but I read the 3rd and 4th back to back. but that doesn't count as one book lol. hmm recently I read The Lions Game by Nelson Demille which was 944 pages.