From what i've seen, you don't get those unless you are doing S convoy/plunder. SS won't drop these at all.

Atm in my server i'm running 1 SS plunder and 2 SS convoy daily (i use 6 ingots for the second one, best ingots/coupons ratio in the game atm, close to 6x). I've never seen any books since the day i stopped doing 1 S convoy daily.