After recruiting Hugo, you can give him your Old Books and read them in the Castle's library on the third floor. In the Japanese version of the game, while reading these books, dataminers years ago found that a seemingly unused crudely-drawn sketch of the game's cover artwork was loaded into the VRAM. In reality, this sketch was not unused, but was actually part of an obscure Easter egg: a secret 9th Old Book. While highlighting the 8th Old Book titled "The Kalekka Incident", press Square, Circle, Triangle, and Cross at the same time. This will unlock the hidden 9th volume entitled "Hirotaka Development Diary" (presumably referring to the game's producer Hirotaka Ishikawa). A very rough English translation of the book's first four pages can be seen below:


The last two pages of the developer diary featured the crude sketch of the game's cover art alongside a request to mail in your opinions on the game to a listed address that is presumably somewhere in Kanda-Jinbōchō in Tokyo (Note: this full page has not been translated). It's not known if this secret is in the Sega Saturn version of the game, but it is confirmed to work on the PlayStation Portable port, Genso Suikoden I & II. However, this port has the last two pages (including the sketch of the cover art and request for opinions) removed, reducing it to four pages instead of six.