User blog comment:Miskos3/Fairy Tail Chapters 532 & 533, Review/@comment-7070784-20170422053208/@comment-1359322-20170422124018

The book according to Lucy is advance life bonding magic, so it's something like an artificial life Zeref create to revive Natsu. Zeref creates his demon by using the same method. The book of END is probably just the most advance version of the Life bonding magic Zeref wrote after countless experiments through the Etherious and corpses like Larcade.