Are you asking about the AI controlling the spells selected by your casters during/after the scripts are up, or about he spells used by fully AI nations or the spells used by independants? Not sure if the first 2 are the same (depending on research available, of course), but we know the last one is different.
All we know about independants is that they have a small fixed set of spells available to cast -- this "grimoire" it's referenced in the progress logs from earlier:
"
27th july
* Indep grimoire slightly improved.
"
This forum had a thread on the comment at that time, including a short note from K.O.
http://forum.shrapnelgames.com/showp...99&postcount=4
Sill