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The most interesting resource on spells at the moment is
http://www.heroesofmightandmagic.com.
It features a very detailed overview of the spells.
I already posted links to this site in the news.
Now I finally got around to translating it. Thanks to NecroFill for making me do it.
The spellbook has 7 tabs:

Adventure map spells

Magic of Destruction

Magic of Light

Magic of Death

Summoning

Combat spells

Special abilities.
Schools of Magic
Destructive Magic
Light Magic
Dark Magic
Summoning Magic
Just the Facts
- There will be only 40 spells. This does not include the various effects of spells obtained by leveling up a hero.
- Each school will have 10 spells.
- Hero's special abilities will be cast from the spellbook for free.
- Cities are not tied to any particular school, but according to reports, the probability of spells from certain schools appearing is higher than others.