Go find some Iron or collect buffs (in the MMORPG sense) for your troops. Knowledge of the Ether gives you Adepts which can get you:
Body Mana: Haste spell. +1 movement. Mandatory for melee heavy armies.
Nature Mana: Tree Top defense. Combine with Haste for fast movement through forest terrain with a +50% defense modifier. Lets see those vaunted withdrawal horsement handle that.
Enchantment mana: Enchant Blade spell: A simple, permanent +20% bonus for melee units and +1 happiness for your empire. Lovely.
Spirit: Courage. Lets you attack enemies with fear. But the major effect of it is that it increases healing rate by +10% permanently, that I think works on the move as well. I prefer to get Courage from Royal Guards casting their spell, but YMMV.
Law: Loyalty. Mostly worthless.
Chaos: Dance of Blades. Gives extra first strike for one turn. Not very good, but worth getting if you have Chaos from Palace. It'll tip the odds about 5% in your favour. The scenarios it is useful for is that it works on all units, even non-living ones like demons. Alternatively, if much stronger than your opponent it'll make you more likely to win fights without taking damage. If your opponent is much stronger than you (such as throwing axemen vs. a super-hero) then it'll much improve your units chance of scoring hits and slowly dragging him down.
Shadow: Blur - Allows units to ignore enemy first strikes, not permanent. You'll be glad you had this when fighting a blitz Gilden Silveric on a hill, otherwise its slightly less useful than Chaos.
Other Tier 1 spells worth mentioning:
Entropy: Rust - removes weapon promotions from enemies. Wipes the smile from Iron opponents.
Metamagic: Floating eye. FLy this sucker over a peak and you'll see why its good.
If you stick with a minimal magic route then its also worth thinking about what use of buffing spells will get you in terms of mages.
Enchant II: Flaming Arrows - +1 Str to Archery units. Who uses Archery units? Who cares?
Nature II: Poisoned Blade - +1 Str to Recon units. Mandatory for Svartalfar. Few others care.
Body II: Regeneration - Increased healing rate on the move. Very nice.
Spirit II: Hope - The Royal Guard spell. Gives free courage to all untis in the city, give culture and happiness to the city.
Chaos II: Mutation: Do you like to gamble? Cast this on a stack of obsolete units for a chance to turn them into . Very good for the Clan of Embers who have lots of superfluous Warrens units and several breaks in the upgrade path.
Shadow II: Shadowwalk - I think this spell has changed behaviour in some patch I didn't notice, but it lets you ignore some terrain/city defense bonuses. I've been very impressed with it when using Svartalfar as it lets recon units take cities.
Priesthood and the right religion gets you:
FoL: Tigers, tiger cages (with carnival), forest creation.
OO: Tsunami spell - excellent city cracker.
AV: Ring of Flames spell - another excellent city cracker.
Order: Bless, +1 holy strength for next combat.
Empyrean: Revelation - mostly worthless.
RoK: Shield of Faith - +10% strength for all units, any unit, permanent. Not very good, but every little helps I guess. Minimal investment required for a permanent boost.
So pick your favorites out of that list and you can ignore everything else about the magic system unless you want to make a mage heavy army. If you look carefully then you'll notice that the best spells for a player who is only going to build 5 Adepts in the course of a game are concentrated in Alteration and Necromancy.