I think it would contribute an added feeling of adventure and discovery if the monster entries in the Hiergamenon were only viewable after that monster was encountered.  It would be extra awesome if they were only partially unlocked upon encounters with the monsters, but fully unlocked when that monster was defeated.  Also, a percentage indicator would keep some people (like me) playing forever trying to get to 100% (and the associated achievement, if FE is going to support those).