He Tries to Ignore it at First

What sense was there to keep a book with nothing to read within?

The Homunculus tossed it aside and continued the modification of what was to be his new home undeterred.

There the book remained for hours, then days. The Homunculus continued working both above ground and returning often to continue the work on his lair. Either engineering the mechanisms to spread his stone or bringing in the occasional object that his pesky Avarice insisted he bring down with him to keep. Each time he did return, he would spot the book still there and wonder about it a little more.

Eventually, he could no longer ignore it. He created a table next to his throne and placed the book onto it and inspected it again. Truly, it was a wonder.

There was certainly no harm in possessing something as harmless as a blank book, was there?

There it stayed for a few days, before the Homunculus would return again, having come to possess more books to read during his downtime and even a writing utensil.

A blank book may as well be written in it, he reasoned.

His mind was a steel trap, but there was no reason he couldn't write anything down.

He remembered watching Hohenheim writing for a few hours during the nights over many years. He'd catalogue the lessons he had taught him, the days events and his own thoughts. He'd insist that it was a secret that Homunculus would have to keep with him.

Now, he had a body of his own. All of the immediate tasks were complete and he had a lot of time to spend until he moved on.

He put his quill to the paper and wrote out: