If you download paid books, that 1 click button can be addictive and that makes it hard to budget! Remove credit cards from your amazon account and each month, buy yourself an amazon gift card. Once you've used it up, you can't spend until the next month.
This always works well if you share an account with someone. If you both contribute a gift card, no-one is worried about an over the top credit card bill!
(Note, without a card on file then you need at least a penny balance to get free books).