Hurt Locker is one of those, if you need it has amazing information! if you don't, you'll be fine without it.

its well written and expanse on CofD base corebook, with more merits, and tilts and conditions for violence and trauma. it also has ways to not kill your player's character's, but they have a permenant pain or something of the sort. its pretty good.

like i said, an amazing book to expanse on Base CofD, but it isn't neccessary.