I second both prior recommendations.
Also, "The New Love Without Limits"-Deborah Anapol.
morethantwo.com has some good articles regarding poly-but he might want to read through to find the ones pertinent for her-because many are about managing jealousy and the like-which wouldn't be as pertinent to someone on the outside looking in.
Here is my explanation (from my blog) one is short, one is more in depth:
Feel free to use pieces or parts or even use it to create a more personalized written explanation of your own.
Finally-my best suggestion is-once she has a working understanding of the basics (which she could get from the two books (opening up and the new love without limits) give her some articles/blog posts that you find which describe well what your personal dynamic/plans/hopes etc are.