In Settings, there's now the choice of 'Default playback method': 'Streaming' or 'Download'.
Also new in this update is an update to Search, it 'now displays suggestions as you type', plus the usual bug fixes. Note that Audible is a subscription service, with terms:
Enjoy 30 days free, then pay only £7.99/month. You'll get one credit each month which you can use on any of our 200,000+ audiobooks.
That's in the UK, of course, prices will vary in other countries. You can grab the updated Windows Phone and Windows 10 Mobile client here. A couple of screens of it in action: