“The Vector Brush Sourcebook” provides 300 unique brushes for digital illustration. The book, by Emily Portnoi, and its accompanying CD aim to teach you everything you need to know about Illustrator brushes—how to make them, apply them, edit them, and use them to stunning effect. It may at first seem that they are just one limited function of a large illustration package, but the array of effects they can achieve is vast. Learn more.
VECTORVAULT - Vector Art Blog
Your Imagination is the Combination
Looks like a good tool to have to learn more about creating brushes.