The book's first appearance was in episode 24.
It is full with all the information about the history of the 7 heroes and how the powers an miraculouses work.

The book originally belonged to the Great Guardian but is wat stolen by Hawk Moth (aka Gabriel Agreste).
Later Adrien found it in the safe behind his mothers painting. And after that Marinette took over the book. Tikki decided to give it back to the person it belongs to, the Great Guardian.