Can Jehovah's Witnesses SURVIVE? is a very decent guide to the beliefs, practices and errors of the Watch Tower Bible and Tract Society. It is truly disturbing that such a malignant religion has grown so common place. Lots of us know where the nearest Kingdom Hall is. Many of us share the same "Oh God, the Jehovah's are at the door HIDE!" experience. They are not your tiny band of Scientologists or your less harmful group of Latter-day Saints. This is a creepy, controlling organisation that treats its members incredibly badly.
Sure I remember when I was younger hearing my family discuss someone they knew who had been converted in the sort of tones one discusses someone who has died. I know lots of people are aware that this is a dangerous group to get involved with. But too many still fall for their lies.
Next time you open your door to the Jehovah's Witnesses beware. But also do as Trevor Willis suggests and offer them an a friendly face and try to let them know you want to help them if they ever need a person to turn to. There is no good to come of trying to argue theology with a Witness (though I did enjoy it when I did it back when I was a smug A-Level student. I nearly turned one! I'm certain of it). Try instead to be their ticket to freedom.
A decent book on a worrying subject.