In-Game Bestiary

I don't mind wiki.
I do use it a lot since is got info in almost everything in the game, if I need something and I can't find it I use wiki.
Your idea sounds good.
And actualy that may make bad players better ( I did said "MAY") ^^

An in game bestiary is a really nice extra to have in games if you're into finding out the background behind certain parts of the game. I wouldn't want something that is text heavy however, and it should have some use in the game. A libra/scan type ability to gather information with could work, or simply killing the monster to get extra info similar to other games. NM's should be immune to any sort of libra/scan as it is in past FF games, but you could get background info on them as a short text paragraph plus a record of what dropped from them recorded somewhere once you defeat them.

If your having a libra/scan /ja in the game it should be design so that its essential to use it get basic info on the mobs you're fighting e.g. you can't see a monster health bar without using it, and maybe after a large number of uses you get the exact HP, but you don't even have to go that far. Other info such as elemental weaknesses would be really nice to have, and it would mean you could punish people far more for not using the right spells on mobs since they have info available if they want to get it.

Going back to NM's, again you shouldn't be able to get any sort of info off using a /ja , but maybe sets of quests, talking to notable NPC or finding hidden scrolls could reveal possible weaknesses (in forms of riddles or puzzles so its not completely served on a plate to you) and basic stats of NM's. This would allow SE to make far more gimmicky strategies than those required for the likes of Odin and DI claiming PW and AV (which they'll undoubtedly be putting into FFXIV anyway) without infuriating people by giving no hints whatsoever on what these required strategies are supposed to be.

Going on the above idea (in the third paragraph), someone once told me a long long time ago that once you'd killed an NM a certain number of times that it would no longer check as impossible to guage, but would check as it's actual level.

That was absolute *** of course, but perhaps libra/scan/bestiary could show nothing on NM's until you've beaten them a certain number of times. Although that kinda stacks on with my first post in this thread.