That's why I didn't get the "free" book. Many, many companies offer 30 day free trials based on statistics of how many who sign up do not subsequently cancel the service. I suspect I would be like many and forget to cancel and begin being charged. So, I very rarely accept free "introductory' offers of recurring charged services.
Please understand, I am not singling out Amazon. I'm a big fan of theirs. Bought the first Kindle they made, am a Prime Member, and have an Echo. Love their products and services.
Just don't usually sign up for "free introductory" offers.