User blog comment:Miskos3/Fairy Tail Chapters 532 & 533, Review/@comment-7070784-20170422053208/@comment-24489996-20170428034919

Just some food for thought:

We have two articles called Writing Magic, and a spell under it called, Letter Re-Arranging - both of which have a single user, Zekua Melon. He used the latter hide the true message he wanted for his son in the book, which is later revealed under the right circumstances. All Lucy did was decoding and unlocking the given requirements needed for the message to reveal itself; I should also note that neither Writing Magic nor Letter Magic are her specialty, as we don't have her listed as such. She doesn't use it in any way or form in the manga or anime from what we've seen. Though she has some knowledge of it to a certain degree, she's not on a level where she can be considered a user. Thus, her decoding and rewriting the information in the Book of END will possibly be next to impossible, especially when there's so much information in it about Natsu.

As Gray stated, she and him passed out from simple trying to read its contents when the information came out like a raging river, and it was impossible to decipher and change it all. Even someone of Levy or Freed's caliber would find it difficult to change even a part of its context because of how complex the book is and how much information is contained. So I highly doubt that Lucy would do anything significant with the book's contents; not sure why Mavis gave them the book in the first place. Even if Lucy had her Gale-Force Reading Glasses to assist her, it's too much complex information to work with. A single mistake in altering its information can cause dramatic changes in Natsu's being, thus the possibility of doing more harm than good.

So yeah, tl;dr, though she's an aspiring writer Lucy isn't really that savvy in the area of changing a book's content, especially if it's a book written by Zeref about his most powerful creation.