The Mage's book is a book held in place of a shield that is a reward from the Mage Training Arena minigame. The mages' book requires 60 Magic to wield.
A mage's book can be obtained from the Rewards Guardian in exchange for 500 telekinetic, 550 alchemist, 6,000 enchantment, and 500 graveyard pizazz points. It is currently the most expensive reward available from the Rewards Guardian. Though a very slow process, the cost of the runes needed for the required pizazz points (While using corresponding staves at the minimal level to cast all Mage Arena spells) will earn you a very large profit.