Be careful with "free" Kindle books. I signed up for that and after a few months, they started charging my VISA for a subscription. They must have got the account number because they are affiliated with Amazon and I have an account with them. When I called, they told me that the free part had expired and that if I didn't cancel it automatically started the subscription. Anyhow, watch your VISA bills. It was $9.99/month.
Watson