Cancel any time. Looks like it would be a great service for heavy readers. Only downside i see is that you will never own these books, and you wouldn't be able to revisit books you've already read unless the subscription is active.
Unfortunately its not the entire kindle library, but 1 million books should cover all the good ones.
Link to the FAQ: https://www.amazon.com.au/b?node=4483857051
You beat my by 2min :) I know that is a long time.