The most helpful book I've read has been Taking Charge of Your Fertility.

Why:
-Insightful information about how a woman's reproductive system works, what the signs of fertility are, and how to interpret them easily.
-I learned things I didn't know and that many other women didn't know, such as the stages of ovulation and how to tell what is happening when and why it's important.
-Charts to help you keep record.
-How to prevent and achieve pregnancy using these signs.
-Each section had example charts on how those signs can be documented.
-It's not just a charting reference. It's a full fertility reference with a charting option.

Mini rant: Every girl should be given this book for their first period (if they're emotionally able to comprehend such a thing) so they can learn what their body is doing and why. It would be empowering to them to understand their bodies. Too many girls are so grossed out by the thought of talking about such a normal and natural function for no reason. And also, they'd definitely read it because tweens are naturally curious about sex and will go so far as to look up "penis" "vagina" and "breasts" in the dictionary so there's no worry that they'd never open it. This would hardly give them the vapors and would open a dialogue between parent and child. My 42 year old friend asked me about cervical mucous today (came up in TTC conversation) and was shocked that EWCM meant ovulation. And she told me "I know I have a vagina and I've been around a while, but ewwwwww" after she read about the different types of cervical mucous. Women shouldn't be grossed out by their nethers!