Edward had a certain respect for books. When he saw people blatantly disrespecting books, he makes sure to correct them. Correct them violently that is.

He made sure not to crack or bend the bindings, and he never spilt anything on the delicate pages.

But everyone has one flaw.

Whenever he borrowed a book, he took it and returned it in pristine condition, except for one thing.

The Doggy ears. Every book he ever borrowed was returned with at least one corner of one page bend downwards. The library ladies had gotten used to this odd fetish, but instead just smiled kindly at him when he asked that they not fix it.

Many of his friends at Central often wondered why he left random pages marked by doggy ears, but never asked, for fear of his wrath.

But Roy Mustang never asked.

When he saw Edward in the library flicking carefully through a book, and then bend one of the pages carefully, he never asked.

Because if he did, Edward would stop.

And then how would he find the books that Edward had hidden in certain shelves?

Then how would he find the love notes that Edward had slipped into those books?

Yes, everyone has one flaw.

Edward's wasn't doggy ears.