[Joyrock]

This might not even be a book, I used to read a lot of short stories, both professional and not, but figured I'd give this a shot. All I remember is that the magic users sacrificed parts of their bodies; small bits of skin and hair for minor spells and even so much as a whole fingernail for some bigger ones.

[Nicodemus]

That would be the novel Hart's Hope by Orson Scott Card. One of my favorite authors, good read.