Something like level 59 with a full alliance and a level 99 in the group. Using an Emperor/Empress band actually knocks this down to level 40 and a decennial/novennial ring pushes it down to 30.
If you stick to a single party of six, even with a 99 present, you only have to be 29 without any Dedication effect.
Now halve all of these again for the double exp event.
tl;dr An alliance spreads exp more thinly that you have to be a certain level to even get 1 exp to get book credit.
Edited, Apr 7th 2013 12:53pm by Raelix