The novel's description starts out with: "A Song for everything, and in everything, a Song." Such is the essence of my magic system, distilled by Renee Swift into a single sentence. Magic users are called muses, and they work melodies in their head in order to have an effect on their surroundings.
Renee also indicates that muses "exist to inspire creation." That is to say, they don't creating something from nothing. Rather, they make use of what already exists - perhaps altering its form or composition - and inspire it to follow the tune the muse plays.
In this first book, we only discover the basics of the magic system. I've taken care that while the system is expansive, it has a rigid set of rules that impose logical limitations. I'm sort of learning from the great Brandon Sanderson here, as I've always admired his magic systems (especially in the Mistborn series). I remember reading a quotation from him concerning magic systems where he was responding to a bit of criticism concerning using magic "rules." Isn't magic, after all, supposed to be about going beyond petty rules? Brandon responded something along the lines that it's his rules that facilitate him in making the magic truly magical. In exploring and pushing the boundaries that he set up, he makes the magic both understandable and amazing. I've attempted to follow the same sort of pattern in my own system.
The key is, magic users have to be clever. They can't just wave a wand and have all their problems solved... yet they can use magic to solve their problems. One of the dangers in writing any fantasy story with magic lies in using magic as a crutch to get your characters out of tight spots. Yet, playing with magic is the fun part, in my opinion, so it becomes a delicate balance.
Having a musical base, I think my magic system develops an intuitive feel. You don't have to have been a muse to understand what they're doing. I borrow a bit from my gaming background here, and use one of my favorite games to help me conceptualize. In Ocarina of Time, Link plays on an instrument (the Ocarina) to cast his spells. In Fates' Motif, the instrument isn't corporeal, rather more of a mental visualization. Each muse, therefore, can have his or her own unique instrument (or not, as the situation warrants). Some instruments may find certain songs easier to play than others. Some may have more carry, or more precise notes. A true virtuoso would likely be able to construct just about any song on their visualized instrument. Similarly, it takes practice for a novice to improve.
Simple concepts have simple notes. Singing a tongue of flame, for instance, out of air isn't completely difficult for a beginner. Fire is a pretty simple concept. Trying to sing a tree into existence, on the other hand, is pretty complicated. There's a lot that goes into a tree, and it has to work... that is, live. Modifying an existing piece of wood (especially not living)? Now that's a bit easier.
Additionally, a melody can only carry a finite distance, depending on the medium, as you might expect. Hurling a fireball is a bit difficult in my system, especially over a great distance. The further you get from the instrument, the softer the noise. Unless, that is, you happen to have an amplifier, or have anchored the tune in something that'll hold the notes. But we're getting a bit advanced now.
The magic system was a lot of fun to play with, and write within. Who doesn't like playing with magic? I believe that enjoyment shows in the novel. There's obviously a lot more to discover in future novels, but I like to think that the basics of this system will seem pretty intuitive to even the the most casual fantasy reader. I didn't want to construct a huge, complicated system that leaves you making diagrams and scratching your head. Rather, I wanted the reader to feel that they could create magic right along with our characters. Hopefully, I succeeded.