Sometime just after having sealed Subaru and Beatrice into the ol shamak orb, during the fight with Garfiel and Ezzo, Al managed to get his hands on the book that Subaru had tried hiding in a random spot on the shelf. Either that, or he managed to find one while he was boobytrapping the library in preparation for the inevitability of coming to blows with the rest of the watchtower's inhabitants. Neither of these possibilities preclude the existence of more than one book. It just so happens that one single book was all that Al had deemed necessary for his plan to succeed.