The book is heavily damaged by fire, rats, and other ravages of time.
From Min Feng, based on cryptography:
Looking at it for a little while, you find seven different "things" that seem to frequently recur at the beginnings of lines. Not all the beginnings of lines have a recurring "thing" like this, but about 2/3 of them do. Five of them are more early-ish in the notebook, two of them are more late-ish. You expect they are names, and the lines are like notes or items that pertain to each person. One of the names always has a small symbol after it.
(It's all written in the same hand, but you think it was not all written at the same time; the ink is not always the same.)
Follow-up from Min Feng, based on KS: Jade Taiga:
The symbol looks a little bit like a "5", and you wonder if that has anything to do with one of the old Taiga legends about the King of Five Grains, supposedly one of the first Taiga rulers, who taught people how to farm.