Audible is such a scam, I'm honestly shocked. They make it so that you buy "credits" that get you the book but you're not buying the actual book. Your monthly subscription of $15 goes toward 1 credit, or 1 book.
Once you start a membership you can only "pause it" for a few months. If you cancel it you lose everything you paid for, you can no longer access anything in your library. It's all gone. Audible is just one giant sinkhole for your money. Save yourself and go buy a real book or download an MP3 that you can keep forever.