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 "Revised Edition". By picking up a rarer version of a currently existing weapon form, you can scan its ‘rarity tag’, a numeral code unique to that weapon, and upgrade the weapon form with its new stats and, to a lesser degree, appearance. The rarer the version of the weapon form, the more powerful it is.

Trivia

 * In the Web Novel, Marin wielded the Vassal Book.
 * In the Web Novel, the Vassal Book increases magical aptitude similar to the Vassal Staff.
 * The Vassal Book cannot copy monsters which have a book-like form.