Toni Weschler is the author of Taking Charge of Your Fertility: The Definitive Guide to Natural Birth Control, Pregnancy Achievement and Reproductive Health, which is pretty much THE book for people who want to chart fertility, and the book I used to learn how to do it well in my 20's. She also wrote a great book about menstruation and charting for teen women, called Cycle Savvy: The Smart Teen's Guide to the Mysteries of Her Body. She's an amazingly dedicated and energetic person who also just happens to really, really like chocolate croissants.
Charting your cycles doesn't have to be about natural family planning. Even if you're not trying to become pregnant, or aren't looking to use charting as a primary method of birth control, there are a bunch of reasons charting can be a big benefit to you.