Thanks, Dpeg.
A way to balance it is to enable 1 level of spells for each 3 levels of evocations, so you would need level 3 evocations to cast level 1 spells, 6 for level 2 spells, 9 for level 3 etc. until 27 evocations for level 9, making casting the later spells much harder. (After 15 the cost becomes quite meaningful.)
In a way this suggetsion highlights the missed potential of Nemlex, because of the random nature of cards, a god that was designed to be a swiss army knife became a god of spammed summons and a few stacked broken spells, while this suggestion gives evokers a huge choice of single use effects, making evokers the most versatile yet complicated characters due to the amount of strategy needed to play them well.
Does it really break the barrier between melee and casters? The conjuration books are pretty much the same as more versatile weaker wands, the utility can be found in scrolls and potions and the better spells in rods and the summons are fun in general and fit much better with melee builds than most casters. Fighters can anyway always train magic as well as fighting which breaks this gap. Being able to cast spells only once is a huge difference, most casters cast the same spell thousands of times during the game, it makes fighters the same as casters in the same way that rods and wands do, which fits in with evocations.
I do feel that it does solve a few issues, at it stands now there are very little differences between one fighter and the other, the choices come down to gods and weapon type and heavy or light armor. Eventually the weapon type will have a bit more meaning once the weapons special attacks are implemented but the choices remain the same. The skills are trained in a very linear manner, you turn off weapon at a certain point and leave armor/dodging and fighting on.
My suggestion makes every level of evocations meaningful, so fighter types have to decide, do I train evocations one more level or leave it and be able to evoke weaker spells but have stronger melee skills or do I compliment my melee with normal spells and be able to cast spells at will but be unable to wear heavy armor. There is also the choice of what spell books the player carries with him. Choices are good and make the game richer as long as they add to the game play.
Trog would naturally dislike using spellbooks in such a manner so the suggestion wouldn't make the strongest (3 rune) god stronger and would still leave a subclass of pure fighters. (Trog should put the user under penance to avoid abuse)
It would make most characters a tiny bit stronger by being able to cast level 1 and later level 2 spells just from evocations natural growth, but as the early spells aren't really that great it isn't a big deal.(carrying around a book for one casting of summon butterflies or raise skeleton won't break the game) Another thing to remember is that most wands are better than conjuration based books (if each spell can be cast once) and the early level charms won't break the game as they are quite weak. Consider the fact that you will need level 9 evocations to cast mephic cloud.
Imagine a scenario of a player facing a tough encounter he has level 9 evocations and sees a spell book nearby, the player must decide is it worth it getting the spell book and hopefully get a spell to deal with the threat. He decides to try the spellbook and it was a book of flames, he happily casts conjure flame and runs for it. Maybe he had level 12 evocations and it was a book of necromancy, so he casts animate dead, etc.
Spells that might be problematic:
Tornado, fire storm, ice storm.- to be able to cast it the player needs level 27 evocations and he can only cast it once so they lose a bit of their power.
Revivification: the biggest problem spell in my eyes, maybe just disallow it but in general it's a problematic spell. (even for casters)
Mass confusion: requires level 18 evocations, it's nice that it will be at last used.
Deaths door: I feel is balanced with the additional requirements of level 10 necromancy, combined with level 24 evocations.
The various summon spells: These in my eyes are the most problematic spells(after revivication) as meat shields are powerful for fighters but most dangerous encounters will take out the weaker summons instantly if they don't ignore them and Okawaru and shining one don't like deaths of summons, so should be ok.
Spells that will be commonly used:
Fulsome distillation: ability to identify one negative potion or create potion of mutation, seems ok.
Summon butterflies: very strong but pretty much any non heavy armor caster could cast it by the time you find it.
Blink: nice but controlled blink scrolls and items with blink would do the same.
Cure poison: potion of healing is better.
Repel missiles: depends on evasion to be effective and one time use gives it limited effectiveness.
Swiftness: a weaker potion of speed
Abjuration: summoners are broken this is a good spell for every player to be able to potentially cast.
Tukimas dance: rare spell that would go well with melee characters.
Raise skeleton: nice defensive spell, not really over powered.
Animate dead: a great defensive spell but with only one casting will animate only 2 or three corpses.
Most of the higher level spells are fantastic and are better than potions but the high evocations skill required to cast them will balance out their power.
most of the spells are fun but not really as powerful as scrolls and potions are, as they are designed to have more powerful effects to go along with their limited use (consider the fact that wand of fire can cast multiple castings of bolt of fire a level 6 spell), it would mainly add a bit of color to non casters and make evokers much more fun and interesting to play.