Grimrock

In some games there are monster libraries about monsters you encountered. Maybe such a thing is possible in-game as well. Let's say you encounter and defeat a new mob. It gets a new page in the library with additional data depending on how you defeated it. If you tried different physical and magic attacks, it'll make summary which weapons and which attacks were most effective. Getting library fully filled with all its secrets for let's say 90% or better gives you an achievement "true hunter", which adds a random double attack due to insight knowledge of mobs.

I think it would be cool, but I also think it's not a high priority thing.

Monster libraries are awesome... when engaged in a lengthy RPG with a vast weapon/magic system. It would probably take 1 year minimum to create a custom Mod that added new monsters, weapons, and magic while offering a great lengthy game experience. Only then would we likely find a monster library beneficial. As the game environment stands for now, it isn't very valuable other than extra reading (lore), since all monsters essentially die from a hard hit, a Meteor blast, or Dispel. Not much else to say about that atm

Monsters are already detailed in the game's manual.

Not all the monsters are listed in the manual and I don't think the single paragraph provided for the monsters that are listed is as much as those who would want a library would be looking for. At the very least the Zarchtons have a lot more information about them already written up if you translate the book shown in the Nexus. You get several paragraphs about them in that.

This sounds like something that could be cooked up with John Wordsworth's notebook mod. I recall that Planescape, and Witcher both had these.

I believe the term the OP is looking for is a "Bestiary."