I like the idea of the spellbooks containing a particular spell. And perhaps if you had diplomatic ties to a nation you could give it to them as a gift, which would be especially useful if the particular spell was one that wasn't in a spellbook you possessed. If not, you could always sell it.

A scroll would work nicely as a magic item; grant access to a cool spell, but only for the particular Sov/champ who has the scroll equipped.

Also, wands as a missile-damage ranged attack that do damage of a particular elemental type (and maybe work off intelligence instead of strength?) would be interesting, so spellcasters still have some power even once their mana is depleted.