The Immortal Slaying Ghost King's Heavenly Book Illusion was a skill in the game. Its specific attributes depended on the game's settings. In the game, delusions could provide additional damage output and increase the character's survivability. Therefore, if you wanted to increase the attributes of the Ghost King's Heavenly Book, you could consider maxing out this skill. However, it was important to note that whether or not to max out the skill still needed to be decided according to the actual situation of the game.