Find the Neverending Notebook in the secret tower and add enough text to the file located in the games directory so that it is longer than 2 pages in-game. You will need to restart the game after adding text to this file.
File is here:
[...]/Steam/steamapps/common/The Longing/The Longing_Data/StreamingAssets/books_plaintext/notebook.txt
According to the games Developer on the Steam forums this is the official solution to get this achievement:
https://steamcommunity.com/app/893850/discussions/0/17456472...https://steamcommunity.com/app/893850/discussions/0/21397143...https://steamcommunity.com/app/893850/discussions/0/17456472...