Vassal Book

The Vassal Book is one of the Eight Vassal Weapons from Glass' World. The person who possesses the weapon is called the Book Hero. The book holds a blue gem that can absorb materials and the ability to transform into, presumably, other book-based forms. The current Book Hero is Ethnobalt.

Overview
The Vassal Book is traditionally held by the Master Librarian of the library rabbits but this is not always the case. The Vassal Book was first seen in the possession of Kyo Ethnina. At first, it seemed like Kyo was a suitable fit for the weapon, however, Kyo was a vanguard of the waves and eventually used the Vassal Book to try to destroy worlds. As a result, the Vassal Book despised Kyo and tried to resist him. Eventually, Naofumi and the other heroes were able to defeat Kyo and liberate the Vassal Book. The Vassal book then floated away and even played back Kyo describing his evil plan to leaders of the Label.

During the Heroes fight against Miyaji, the Vassal Book and the Vassal Mirror appeared just in time to save Naofumi and Itsuki's lives. Both weapons tried to make Naofumi their wielder but only the Vassal Mirror was successful. Then the Vassal Book flew away to find its new wielder. It eventually selected Ethnobalt as its new wielder and led him to the fight. With the two new Vassal Weapons, the heroes were successfully able to defeat Miyaji and his companions.

Power-up Method
The power-up method for the Vassal Book is unknown

Weapon Copy
The Vassal Book gains new forms in a manner slightly different from other Vassal Weapons. It works by copying books instead of monster materials. Each book that the Vassal Book can copy has a different quality and category rating. The contents of the book affect what will be unlocked.

Trivia

 * In the Web Novel, Marin wielded the Vassal Book.