User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-71.222.52.171-20151209065359/@comment-1359322-20151209073348

The book is also Natsu. When Zeref shot the book, Natsu was clearly affected by that attack. That's why Igneel told Natsu not to destroy the book, the book is destroy, Natsu dies. As to why not to open it, not sure, maybe it states down the creation of E.N.D. and his history. Igneel didn't want Natsu to find out his identity.