Guardian Wind is a unit spell. It can be used either inside of combat (for a mana cost of 10) or outside (for a cost of 50, plus an upkeep of 2 mana per turn; this is for each unit you choose to protect).

It should be in your spellbook under Unit Spells; you wouldn't have been able to acquire the spell had you not been able to cast it (i.e. had at least one book in Sorcery).*

So what could've gone wrong?

1) In the spellbook, you weren't looking on the Unit Spells page.

2) You never saved the game in which you acquired the spell; perhaps for some reason you decided to reload and try again.

3) You misremembered (or misread) which spell you acquired; maybe it was the Life spell Guardian Spirit, for example.


*To clarify, you can't automatically acquire the best spells with a single spell book. To receive common and uncommon spells (via exploration or diplomacy) you can get away with one spell book in a given discipline; for rare spells, two; and for very rare, at least three. This of course doesn't apply to Arcane spells, which everyone eventually learns anyway.