I thought this was mentioned on the 1.0 forums before and had a community rep reply on the thread. That we would get an in-game Bestiary or one that will be put on the Libra Eorzea.

I would like to see if one is implemented as I am farming items for crafting, and I would like to see what monsters drop what so I can farm and craft items without having to rely on the Market Boards or purchasing them in the Guilds. We have the Gathering Logs, all we need is a drop item monster log...