Spiritual-magic systems?

[efreysson]

I hope no-one minds YET ANOTHER magic system thread. It's just that while I do intend for magic to exist in my fantasy world, actually creating one kind of terrifies me, partly because of how to fit it into my setting and stories without messing things up.

For my setting and stories to work, I need magic to be rare, mysterious and with some very definite limitations. Rare and limited enough that hardly anyone thinks to bring in a sorcerer to solve a problem or win a conflict; I need all that to be down to old fashioned cunning and swordwork.

Also, given that I have a widespread secret order devoted to battling dark powers and preserve knowledge, I can't have sorcery be a simple matter of reading enough books, since then, LOGICALLY, my warrior-scholars would simply open schools and crank out fireball-flinging superheroes to stop the bad guys in a few months.
I've finally fallen upon the idea of making it spiritual to some degree (or whatever the concept is called): That rare individuals with a very special sensitivity can channel power from the spirit world to work minor 'miracles', for lack of a better word.
What is kind of stopping me for now is that my world's religion reflects my own: A vague idea of a spiritual afterlife, with the deceased watching over the living, but with no real idea of who or what is in charge of it. Even so, I'm open to the idea of bringing in positive spirits (angels, again for lack of a better word), who grant power to mortals. Or just have the spirit-realm be composed of an energy that sorcerers can manipulate. As the flipside of that, I could then have the bad sorcerers draw power from demon-like entities.

Could someone give me examples of this kind of thinking? Real or fictional? Or just make suggestions?