The last part the Lord of the Rings book was not the celebration of the victory over Mordor.
It was titled “The Scouring of the Shire”, and covered the Hobbits having to deal with the ruffians who had taken over the Shire while they had been off to war.
Yup Sauron - I remember that. It's been years since I read the books.
I believe in the extended DVD of Lord of the Rings, Peter Jackson discusses why he didn't include it in the theatrical release of the films. The scene where the Hobbits are looking into the Mirror of Galadriel, and they see the Shire all burned and people taken captive was a filmed scene about this that was not included in the release.
https://lotr.fandom.com/wiki/Scouring_of_the_Shire