I'm pretty sure it's already been asked but I'll resubmit this idea.
Multiple spellbooks for classes that can swap spells at shrines/taverns. I multibox and have my buffs arranged across my toons for the best configuration. However, on their own, the toon usually lacks some of the generally useful buffs that you would normally carry. I'd like an extra spell book or two so that I could hit a shrine or tavern and swap to a more group based book than the one I would use in my "soloing". I'm sure it would be useful across the board for most players with toons that can swap spells too.
This is what I meant...