Details of a magic system
In a comment on my previous post about magic systems James asked a few questions about about the possible origins of a system, and the amount of details I would want to see in it.
Trying to find an answer to those questions was one of the reasons I wrote the last post; to help organise my own thoughts around magic systems and thus help me create better ones for my own fantasy settings.
The origin of the structure isn't really that important in my opinion. I care more about the actual structure and how it has (and will) affect the setting. If the magic is an inherit part of the world, power from another realm, or something given to people by their Gods does not really matter.
If the system is carefully laid out with mathematical formulas I, as a reader, don't need to know that. I want enough information to predict results and options within the system, but an approximation is enough in that regard. The difference between knowing the character can create a 10" fireball and a 11.5" fireball is not that important (in most cases), knowing the character can create a fireball is the important part. For the author it might be a good idea to lay out the rules with all the small details made clear. I know that I at least have an easier time keeping track of everything, and figuring out all the possible implications of my system when I have it clearly written in front of me.
Balancing the amount in information, and figuring out how to give is a very difficult task. The reader doesn't want an info dump with all the rules, but he should learn enough to anticipate the limits and available options set by the rules. I've noticed that it is often solved with the character learning about the system alongside the reader (e.g Brandon Sanderson in the Mistborn books or J.K. Rowling's approach in the Harry Potter books), but that option is not always available. I cannot think of a book that does it any other way at the moment, without info dumping, but there has to be some out there.
You can of course avoid all of this by making magic a secondary part of the story. If it is never used to solve any conflicts it does not require the same amount of exposition.