User blog comment:Miskos3/Fairy Tail 534: The Door to the Vow, Review/@comment-24039234-20170429161846/@comment-7418318-20170429223324

In a way a Celestial Spirit Mage links their own life with the Celestial Spirits (hell, we even saw Lucy almost merging with Loke's energy back in the Loke arc) and we know that Zeref has at least a little bit of knowledge about Celestial Spirit Magic, since it's part of the Eclipse Gate.

Do you think that he might have used principles he learned from his research into Celestial Spirit Magic to create his books and that's why Lucy is able to meddle with it?