Now I don’t have like, a solid canon basis for this, it’s literally just my fun hc, but I like to think that on some level Atsushi is the book. Like, if you’ve ever watched Dr Who, in one of the specials there is a weapon that’s so powerful it develops its own consciousness, and I just think with the way Atsushi is the supposed wayfinder of the book, in DA shibusawa said he was the envy of all abilities/users, and he said his own regenerative ability doesn’t heal wounds but negate or rewrite them, I just like to think it would be kinda cool if he was a manifestation of the book itself