User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-5303512-20160927112257/@comment-84.197.239.101-20160927131511

Most likely it depends on Zeref's intention.

He wanted to dispose of Mard Geer so he ceased to exist, but Zeref likely wants Natsu to stay alive... at least for a while longer until he fulfills his purpose.

It is possible that Zeref's shot through the book triggered E.N.D. but perhaps it was due to the fact that Natsu faced off against Zeref with a genuine intent to defeat if not destroy him and this intent awakened his slumbering purpose.

I do wonder if that book is still able to influence Natsu, for example Zeref "rewriting" the content to force Natus to become more serious in defeating Zeref.