I'd go with Draco for sure. If you get lucky and hit all the cooldown reductions on his a2 and a3, Duchess can probably get the rest.

Without knowing anything about the rest of your roster, I'd say book Draco until his A2 and A3 are maxed, then Duchess, then Rae. Stag Knight is also worth full books, he's an epic so different books anyway. But depending on your roster and your priorities, maybe doing it differently could make sense as well. 

Thanks. 

I have put 5 books on Draco. 3 on a1 and 2 on his a3. If I select the last 4 together they probably go all in a2. But is it worth it?

Nah, selecting books together can still see them end up spread across several skills. But yes, it's worth it, he'll put a lot more poison on the CB and Dragon once you have that reduced cooldown on the A2. For some other content, he may already be fine as it is with the A3 maxed. 

i vote duchess, just amazing.  if lucky only 3 books needed.  at  most 8