subscription or just the usual month to month?

The nice thing is there is a large back catalog called 'plus' that probably has a large number of books that you'd like to listen to- those you can add to your hearts content to your library. The only qualifier is that your subscription must be active to listen to them.

Also they will occasionally have sales on credits where you can buy bundles of credits. Stocking up on those is always going to be cheaper than buying retail price for a book once you run out of subscription credits if you find yourself out-listening the one per month model.