[SixOfSpades]

The fact that you mention Aerie makes me wonder if you're trying to buy Priest spells. When you go into a Temple and ask to see what spells they provide, they are not offering to sell you scrolls of that particular spell; they are offering to cast that spell on you. What's more, they won't let you buy a spell that you don't need: Temples will not let you purchase a Raise Dead or Resurrection for a party member that is still quite alive, nor are you allowed to buy a Restoration for somebody who isn't Level Drained. That may be why Aerie's "Buy" button was not lit.

Basic Facts:
Mages (and Bards) get new spells by first finding a Scroll of that spell, and then scribing the spell to their Mage Book. There is a chance that their attempt to scribe the spell will fail, in which case the scroll is destroyed and therefore wasted. Once the spell is safely written in their Book, however, the Mage or Bard can memorize and cast the spell at will (well, provided they have the spellslots to memorize it in, of course).

Clerics and Druids get new spells simply by Leveling Up: When they gain an experience level that allows them to cast a higher level of spells, ALL of the spells for that level appear in their Priest Scroll, ready to be memorized. There is no such thing as adding new spells to your Priest Scroll--except much later in the game, when Clerics and Druids are allowed to choose some "Quest-level" spells, which are added to the 7th page of their Priest Scroll.

Aerie, being a Cleric/Mage, learns spells by both methods. But don't get them mixed up: The Mage Book is one way, and the Priest Scroll is another way, and never the twain shall meet.

You will find a few scrolls of Priest spells in the game: These can not be "learned," by any character. They are simply there for the purpose of spells cast directly from scrolls: Put it in your Quickslot and use it to heal your friends.

The link in my sig will tell you a whole bunch a' sh**.