A limitless system can be observed in the worlds of Harry Potter, and as far as I can tell, Lord of the Rings, where there isn't really any discernable cost to the use of magic other than some willpower and the desire to learn. Though I'm sure Tolkien had some sort of rules in mind to prevent Gandalf from just whisking the One Ring away and doing his thing with it.
Rules to me imply a system of checks and balances to ensure that I can't just MacGuffin my way out of a corner and cheat the characters through any tricky spots. I'm not sure how to write a good story with a limitless magic system such as Harry Potter or Lord of the Rings without taking the opportunity to cheat, so I've been reworking BookB's magic system to be more solidly based in rules, with costs and clear definitions of what can and can't be done within those rules.
The trick of course will be to avoid the system feeling "Gamey", the last thing I want to do is put in a whole bunch of math that doesn't fit and distracts from the story.
So I've spent the last few hours reworking the basics of one portion of my magic system to lay the groundwork of a set of rules. It'll keep me honest, and hopefully make the whole process and resulting story a better read.
2 Blog posts and a good 3 hours worth of writing work done today. Not bad.