An Awesome Passage About Magic Spells

Bonus points if you can tell me where it's from:

"I know I'm slapdash," he was saying, "but there's no need for you to copy me. Always read it through, carefully, first. The shape of it should tell you a lot, whether it's self-fulfilling, or self-discovering, or simple incantation, or mixed action and speech. When you've decided that, go through again and decide which bits mean what they say and which bits are put as a puzzle. You're getting on to the more powerful  kinds now. You'll find every spell of power has at least one deliberate mistake or mystery in it to prevent accidents. You have to spot those. Now take this spell..."

Just lovely.