If all the spells are carbon copies of each other, That is a bit dissapointing. I guess i will just pick one and charge after all the shards for that particular element. You could make it so water has a lot of healing, Earth is defensive, Fire is offensive and Air is the special attacks (Catch the reference to an epic PS2 game and you get free karma)
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They're not all carbon copies. You do get unique spells, like air having haste for example. The problem is these are few and far between. By level seven you might have two or three unique spells per book, three or four identical damage spells for each book and a whole bunch of generic non-elemental spells like protect city.
Which gives you a couple of problems. If you take one of the pre-built sovereigns who have all four books, or create one yourself with all four books, around half your spellbook is filled with identical spells which differ only in name and graphical effect. Like I said, there's no reason for me to cast lightning over flame dart, both do INT damage so the effect is identical. When you have four such spells at each level it feels like half your grimoire is simply spam.
Funnily enough, it's the same problem taking only one spellbook. Due to the generosity of early level generic spells a fire mage will differ from a water mage by perhaps two spells. There's no real sense that the magical schools are different in any way. Unlike MoM, AoW or even Lords of Magic, there's no real sense of "that guy is an air mage" which I think is pretty important when you're going for a spell list system.
Differentiating the schools in some way would help, even adding secondary effects to each direct damage spell (say lightning bolt has a chance to stun the target, flame dart might ignite them) would at least give you a reason to be packing these spells. Pacing out the more generic spells or perhaps even taking a leaf out of MoM's book and having a separate Arcane book for them might also be an idea; reduce the universally available spells to the most necessary (say the arcane armour / weapons, protect city and heal) and have everything else in a separate book.
Another trick from MoM would be to offer different spells for focusing on one school or another. There's no spellbook levels, so perhaps the easiest way would be spell effects altered by spell books, so for example say a fire spell which does 5 x INT damage, with the multiplier reduced by one for each spell book above one (so if you have all four books it's 1 x INT, while if you had fire and water it's 4 x INT).
Finally, more focused themes would help, especially if adding spells. There's already a hint of them in there; it looks like Air has a tendency to manipulate movement for example, but these are diluted by the wealth of generic spells. It might be worth picking two or three aspects for each book and flavouring the spell list around these, so say Air manipulates movement and temperature, so 90% of air spells should affect one or the other, with 20% affecting both in some way. It could be secondary effects (damage might freeze or slow the enemy for example) or even simply cosmetic (casting blizzard turns the tile into tundra).