Well, I know one way to make tomes give + Mana, but your probably better off just using triggers. Anyways, you know the glyph of fortification that can upgrade building armor? If you modify the ability that glyph contains it will essentially research 1 level of the upgrade it is linked to. You could make an upgrade that increases mana by whatever increment you need. You would have to set the upgrade to effect the units that would gain + mana and it can only be used up to 100 times (because upgrades can only go up 100). If you just have 1 hero you could go this route

other than that-use triggers XD