"Taking Charge Of Your Fertility" by Toni Weschler
This book provides an understanding of your body's inner workings like no other, I honestly think every woman should read it. You can get a used copy on Amazon for something like $5 shipped. You can KNOW the day you ovulate, you can KNOW that you're pregnant without a pregnancy test, you can help pinpoint fertility issues if you are experiencing them, this book is just full of things you wish you always knew!
Next question in Health
Can PID (pelvic inflammatory disease) cause you to miss a period?