N.K. Jemisin's overview of why magic (in fantasy novels) does NOT need rules is simply the best. I remember vaguely thinking that the statement that magic must operate by some sort of systemic rules was odd when I read it in Orson Scott Card's "How to Write Science Fiction & Fantasy". Jemisin's analysis delves deeper into why people insist on that - and why they're wrong. As she put it, "Game logic should not apply to magic, because it’s fucking magic."
Read it here: http://nkjemisin.com/2012/06/but-but-but-why-does-magic-have-to-make-sense/