User blog comment:FairyRaven/Fairy Tail Chapter 416: Tartarus Arc: Final Part, Review and Commentary/@comment-5303512-20150129142724

I think that the Book is E.N.D's soul while Natsu is E.N.D's physical body and that they were both separated and that is why Igneel didn't want Natsu to open or destroy the book.

In order for E.N.D to never again rise, his body and soul have to be separated.


 * And that would explain why Natsu doesn't know why he couldn't pass through Freed's barrier. (Though his thought can also be tied to Dragons inside the Dragon Slayers)


 * Why he could eat the Ethernano Lacrima without dying from Poisoning.


 * Why Natsu doesn't know Zeref and that he survived his Death Magic.

As For the Book of E.N.D.


 * E.N.D is able to command Tartarus even in his sealed state and is able to communicate with Mard Geer.