Do they drop with a higher chance in elite than in advanced (and in advanced than in normal)?
Books of dark rituals
You know what? The more I read about this the more I think that the best choice would be to run around on Normal or Advanced difficulty, just to speed things up.
Black Kats appear (supposedly) with a 1% chance. Books (supposedly) drop with a 1% chance. Therefore being able to get the most Black Kats to spawn is the best choice....
So that's either rushing levels on Hard, or rushing levels on Normal... I guess it depends how much of the level is spent at parts that require you to kill enemies or get keys.
While you should probably do your research, I'll go easy on you this time. :P
In my experience, on harder difficulties, it is more likely for more valuable things to drop. However, keep in mind that 5* materials only drop in T3; grinding T2 won't net you a book. I would suggest going to Stratum 6; It's the most likely place to get Books.
For those of us who weren't here last time...what do these books do?
"For those of us who weren't here last time...what do these books do?"
The Book of Dark Rituals allows you to summon Margel, a boss that, when killed, drops the Black Kat Cowl.
Try this Orkorrior -----> http://wiki.spiralknights.com/Book_of_Dark_Rituals
My general impression is that more loot drops in higher difficulty levels. I don't know if that increases the chance per se, but I would assume it does.