You need 5 black books for the final SR limit break, and 5 gold ones for the final SSR limit break. For Advents, getting the gold books is suuuper expensive. You'll get plenty of them from raids and Union events. Hopefully you're in a good Union.They do nothing inside Kamihime Project. Once the event is over, you'll get stuff in Dragon's Providence.
Speaking of which, you might want to do DP to get 500 Alyssa Coins so that you'll get free shit in Kamihime once the cross-over event is finished. Or maybe just 300, since getting 500 only nets you a crappy Dark SR.