User blog comment:Imhungry4444/Fairy Tail Wikia Podcast 10/19/13/@comment-24039250-20131022192447/@comment-24039250-20131023020042

Also the spell "Nemesis" a spell that can create demons from nothing is from Verse 20 from Volume 4 of the Book of Zeref. That spell is just one of many from that specific volume meaning there are more but I doubt there are a lot. The 9 devils we see here may all have just came from that singular book that E.N.D possesses since all we know is that they possess a book of Zeref, not multiple. We will see what happens, since i can always be wrong.