If you're an audio book fan then the dominant Audible system's presence on Windows Phone has probably been a disappointment in the last couple of years, thanks to a 'next gen' client back in 2013 which turned to be a damp squib, with unreliable playback resumption, Bluetooth bugs, purchase issues and more. Thankfully, Audible's developers have returned to Windows Phone and seem intent on quoshing all the bugs, with several 'beta' updates in the last couple of months and one just a couple of days ago. Best of all, their work in progress is public, so anyone can have a play. It installs happily alongside the older, stable client, don't worry.

UPDATE: 18 March 2015, it seems that the beta improvements HAVE now been rolled into the official client, so if you have that installed, just look out for the update hitting your on-device Store.

From the Store entry for the Audible beta client:

Not enough time for all the books you want to enjoy? Download the app by Audible.com, an Amazon company, to listen to books on the go.

This is a beta release of the app for users who want to help test the latest build and are willing to tolerate the bugs that may come with a beta. Please provide feedback to audible-winphone-feedback@audible.com or use the app (Settings & Support > Help -> Feedback and feature request).
This release includes:
- Bug fixes
- Improved stability
- Personalized recommendations
- View similar titles by the same author, narrator and next in series
- Rate and review titles from the app
- Play audio samples in the app
- Storage card support

In the fullness of time, of course, this beta version will be matched by the official, 'stable' version (ironic, considering the number of bugs!), but there really is no penalty for having both on the go at the same time. The interface is extremely similar, with just the lack of bugs and issues to distinguish them.

A few screens of the Audible Beta client in action today:

Browse my library and Audible's content - note the slide out, Android-style hamburger menu/pane - argh!

Playing an audible book and (left) setting the sleep timer, with new, smaller intervals....

Two of the client's unique features, a full screen interface that doesn't require you to look at it (e.g. listening in bed, eyes closed) and a voice control function - this works even when a book is playing via the speakers, amazingly....

Recommended if you like audio books - this client is certainly up with the functionality of the iOS and Android clients now. Grab it here in the Store.