This item allows you to store your spells in it in such a way that it does not use up your entire inventory. It allows you to cycle through available spells using your keys as well. It is dropped from the Arcane Guardian. This is the same as a normal spell book, but it slightly buffs damage and reduces mana cost slightly for your spells that are placed inside for use.
WARNING NOTE: as with the normal spellbook, pressing the hotkeys to scroll through the book can initiate a glitch which causes your instance of Minecraft to immediately crash.