Eiro filled in the blanks of multiple books that he was trying to repair, or rather rewrite, in order to remember what they offered him. The more he did so, the closer the memories seemed to come, but they were always still a bit out of reach.
The Demon figured that he would only be able to truly remember a memory after fully rewriting the book it connected to. The issue with that was that Eiro was unable to fully complete any of the books that he had laying in front of him.
He might be able to guess what happened there and fill the gaps by concluding what happened from the text around it, but for some reason Eiro thought that was a bad idea. As if that wasn't what he was supposed to do with these books.