Ovulation tests detect the surge in Lutinising Hormone (LH), the hormone that triggers ovulation. The test cannot tell you that you are definitely going to ovulate, only that the LH in your system is rising: some women get several surges before actually ovulating. They also cannot tell you that you have ovulated - only a blood test can confirm this.
The best way to use OPKs is in conjunction with other things - charting your temperatures, monitoring your cervical mucus, etc.
Just remember that a 28 day cycle is actually pretty uncommon, don't trust a book to tell you when you will ovulate - trust your body and the signs it gives you. Plenty of sex - 3 times a week will cover it - is the only way to maximise your chances.