hmmmm
another thing

sai is dressed as a ninja on the last page (probably ANBU without mask)
when i recognized it as such something came into my mind.

"in Root, you have no name. You have no emotions. You have no past and no future"

so when he joined root he had to forget everything about it and maybe that book is the remaining thing which reminds him of his old dream? xD

so he's faceless because he is not allowed to show emotions (since the only way to express emotions is with the face in such a book and its a way to identify a person [no name] )