Book of Truth

The Book of Truth is an artifact that symbolizes the Principle of Truth. First appearing in  Ultima IV, it features in the series as a component of certain ritual magics, serving as a crucial component in the initial unsealing of the Great Stygian Abyss. It has been described as a heavy, leather-bound tome with vellum pages and gold lettering.

Age of Enlightenment
The Book first appeared during the Stranger's quest toward Avatarhood, where it lay within the libraries of the Lycaeum. Likely after having consulted Father Antos, the hero found the tome in the wing marked with a "T," and later made use of it in conjunction with the Candle of Love and the Bell of Courage to open the mouth of the Abyss.

Age of Armageddon
It is not known precisely what befell the Book after the Stranger's transformation into the Avatar, although it appears to have been recovered and returned to the Lycaeum, where it could be found in later years. Centuries later, the returned hero had need of it again, that they might reestablish the psychic link between the body and soul of Shamino, whose spirit had become trapped in the Ethereal Void, by enacting the ritual of Reunite Body and Soul. Before the Book would be granted to the Avatar, however, the champion of the virtues had to confess responsibility for the evils that had befallen Britannia, unknowingly acknowledging the greater truth of the Guardian's identity.

Trivia

 * The Book, Bell and Candle are traditional symbolic devices used in the ceremony to seal an excommunication in the medieval Roman Catholic Church. After condemning an excommunicant, a bell would be rung as a symbolic death toll; a book would be closed to signify separation from the church; and a candle would be snuffed to represent the extinguishing of the condemned individual's soul.
 * In Ultima IV, retrieving the Book grants (only) the player's character 400 XP.
 * A modification by Marzo Sette Torres Jr. for the Exult utility for  Ultima VII places the Book of Truth in the Lycaeum.