User blog comment:Omega natsu2/Chapter 413: The Book of E.N.D., Predictions./@comment-172.56.3.122-20141216004353/@comment-7418318-20141218120247

Not necessarily.

For example Igneel might have defeated END's body, but before he got a chance to destroy the book, he was attacked by other Demons or maybe even Zeref himself and due to his injuries had to retreat without the book.

Or the book could have been hidden away in a secure place, while END went out with his body and Igneel was unable to track down and destroy the book after killing the body.

Or END might have forced Igneel to retreat, but took so much damage that it couldn't sustain its form any longer and went back into its book. Kind of like a lethal injury.