I dunno if this is common knowledge or not, but I figured I would post this anyway..!
The game has a bunch of secret spells as I've just discovered -- not the "cheat" spells, just regular old ones. I'm bit angry the manual lies, but whatever. :)
Anyway, this German site has a guide up for it here. The guide is in English but the overview is in German. A translation:
"Those marked with a * signify spells which do not emerge in the Spellbook; they are “secret” spells, which one must find by trying out oneself. Some of them are simple to figure out, for example the rune Fridd (cold) can replace Yok (fire) in some spells in your Spellbook."
I may get super geeky on you and draw these out and upload them at another time. :)
For now this will have to do:
Harm* Rhaa Vitae * Damages opponents within a radius around the spell-caster
Ice Bolt* Aam Fridd Taar * Hurls a projectile of cold
Protection from Cold* Fridd Kaom * Reduces the effects of ice attacks and cold
Slow* Rhaa Movis * Decreases someone's speed
Venom Cloud* Aam Cetrius Taar * A cloud of venom drifts away from the spell-caster
Wall of Ice* Aam Fridd Spacium * Raises a wall of ice in front of the spell-caster

I don't remember if this is a secret spell, but:
Drain Life * Vitae Movis * (might be the other way around, movis vitae. I'm not 100% sure).
It is a very useful spell on hard fights. It damages everyone close to you and heals you in the same time. Kinda like a strong Harm spell with added minor invulnerability from the health gain. The damage it deals is quite high too. Too bad it uses much more mana than Harm.
Also, you can stack multiple Harm spells to deal more damage. For example, cast 10 Harms and you will deal 10 time more damage. The mana drain is ten time the normal too, but Harm uses so little mana that it really doesn't matter all that much. If you have high amounts of mana and mana regain you can keep those 10 Harms up for a minute or so. It is REALLY helpful on those hard enemies like Lich fight and such.
And ofcourse the improved summoning spell, *Mega Mega Mega Aam Vitae Tera *. It needs really high casting skill. Around 100 or so, if I remember correctly.

Awesome stuff. :D