[TOMT] A book I read a while ago, potentially written in a made up dialect about a book or set of rules that are discovered years later and used as a bible?

I’m looking for a book that I remember reading and want to read again but can’t remember the name

It’s a fiction book where a notebook is buried and is found years later. The first half is written in dialect or a made up language.

I think it’s set in two timelines and the world ends, then many years later on an island(?) the book is found and they use it as the way to build their society.