User blog comment:IamJakuhoRaikoben/Chapter 360: The White Inheritance, Review./@comment-4489126-20131115224855/@comment-7418318-20131116083342

Apart from that we still don't know how those book of Zeref thing works.

Maybe he also created blank copies of those Demon summoning books that new creations could be tied to by his servants.

Kinda like the blank Runestones from the first Spellforce addon, which could imprison the souls of slain enemies and then a mage "activated" them, allowing you to summon those slain enemies to fight for you at will.