User blog comment:IamJakuhoRaikoben/Chapter 462: Battlefield, Review./@comment-1776824-20151116230642/@comment-4829046-20151117074136

It is still unknown if END actually means something with the current Natsu. It could very well just be that the book unleashes his Demon curse instead of consuming and corrupting him. Zeref might be against having the book touched because he doesn't want anyone accidentally giving Natsu his full power or something. In the reverse though, it could be exactly what Zeref needs in order to attain Lumen Histoire. It is all assumption.