This is an absolutely amazing book. This is what we never learned in health class. The book explains exactly how women can understand their own bodies and fertility and helps you pinpoint the day of ovulation (well, down to a range anyway.) No, this is not the rhythm method...

Why would you want to know this information?
-To know if you are actually ovulating, and that your body is working properly.
-To know the difference between when you are genuinely "late" on your period or when stress may have simply delayed ovulation. (The time between ovulation and your period does not really change, so knowing when you ovulated can help.)
-If in a monogamous relationship where you have both been tested for STDs, to target your use of barrier birth control to prior to ovulation (and a few days afterwards.)
-To understand and recognise whether the monthly pains and changes are actually indicators of health problems or are your normal cycle.
-To know when you are fertile if you're trying to get pregnant.
-If breastfeeding, to know whether you are still infertile (as breastfeeding is often used as "natural" birth control by many since it stops ovulation.)

Seriously, I never thought I'd be all into "natural" birth control, but every woman should read this.