This is the forum for helping you out if you're stuck or if you want to discuss the nitty gritty details of the dungeons within Mount Grimrock. Warning: forum contains spoilers!
Also note that you will require the necessary skill (in a spell school) to cast the spell.
For example, if you find a Fireball scroll, you won't be able to cast it even if you hit the right runes if your mage does not have the requisite skill in fire magic.
Also, you should know that you don't need to find a spell scroll before you can cast the associated spell. The game manual describes all the runes, and so I think you are encouraged to experiment with different runes until you discover a new spell (without finding the scrolls first). There is logic to what runes cast what spell.
If you didn't do any experimenting like that, then there are a couple Magic Schools in particular that would leave the Mage completely powerless for a long time until an appropriate scroll was discovered.
I think a typical Mage progression is to level up a magic skill until you unlock a "Spell: ??" And if you haven't found the right scroll for that spell yet, then start experimenting until you discover it.
One question, i have already 25+ Air Skill and have found until yet (L5) just one Air Scroll.. i have some ??? in my air magic skills for spells, have already 2-3 scrolls for every other magic line but just 1 for Air ^^
Should i just try some combinations to find spells out, or its a scroll for every spell in game?