
Grimoire synthesis is confusing. You use 3 stones, one of which is lost in the process. Then you have whatever number of slots from one of the stones and can choose from the skills contained in two of them. I'm still not sure which stone in consumed. I think it's the base, but maybe it was the last one. I stopped doing synthesis for a while and just chose whatever stone was useful. I still don't get the equipment bonus. I guess it allows you to equip a weapon your class usually can't use. Sometimes it's seems to disallow the weapon you're currently using though. Some of those skills are handy however. I have a medic with a stone that has a Volt spell. It gives him something to do when his normal attack is useless.
I've been using the Reference option in battles lately. You hit RB and you get a breakdown of the creature's weaknesses, if you've met it before. I used the Analysis spell and items before to try and uncover some of the question marks there, but they don't seem to do anything.