@Thunder-The-Bright

Once per 200 killed black kats (and there you wonder why black kats are so rare :P).

(and to make it a bit more depressing: I rounded up :P - happy hunting).

@Autofire

I can tell you straight away that killing regular kats vs. speedrunning has no no effect on your chances of getting a book (other than seeding the RNG with a different number, but thats a discussion on its own). There is no deliberate mechanic that feeds your performance back into the reward system.

The odds for getting a kat (let alone a book) are so slim that you'd have to take performance data from several levels into consideration. Parties usually don't stick together for longer than a single run (if at all). That means you'd have to add logic to the game that meassures performance on an individual level. You might get the idea of adding a kill counter to knights, but what if someone plays support bomber with shivermist? S/he would attack pretty much every kat, but rarely get any kills. So in order to not penalize bombers, you would have to count attacks instead of kills, but then you have a problem with players not finishing the job.

If you manage to come up with a set of rules that actually produce meaningful statistics, then you have to persist them in the database. That costs storage space and RAM on the database server (even for the millions of dead accounts).

Lastly, you'd have to add logic to the reward system that takes a sketchy performance value into account. This is exactly what OOO is not interested in doing. They make money based on the rarity of items and that means they are highly interested in hammering a percentage somewhere into the configs that limits how many rare items of a given type will spawn.

In short: basing your chances of finding a book on your performance just adds a lot of complexity to the system with no real benefit. So there is no point in doing it.