Well, against my will, (but it was really my choice) I decided to farm some out to possibly find a Book to give to a lucky friend of mine who is deserving of one. I found an interesting trend. So, I went about soloing (GASP! Panda SOLO'D!?!? It was terrible) and I didn't really find any Kats, but then I join a fellow (Zaffs, love me I got you so many Kats) and Kats spawned the second I came. It was the weirdest thing! I did a few runs one after the other with him, but didn't find any, so I solo'd again. Then I rejoined the chap, then AGAIN more Kats. I tried this multiple times, and it kept occurring. Solo one, join, ect.
I'm I just lucky, or is it maaaagic? :3
I found my first Black Kat with someone else and then my second on my own. I'd wager my two pages that being in a group does improve your chances but not by much.