User blog comment:FairyRaven/Fairy Tail Chapters 338, 339, 340: Review and Commentary/@comment-86.66.207.63-20130706094810/@comment-1359322-20130707112725

Zeref is 400+ years old. The Book of Zeref might be a book he wrote few hundreds years ago. I think few hundred years is enough to turn something into an old legend. Also not that it's important anymore, maybe the Dragons knew about the Eclipse Gate and its functions, assuming if that's how they have the knowledge to come to the future in the first place.