Android Candy—Smart Audiobook Player
The Audible app for Android is a great way to consume audiobooks. You have access to all the books you've purchased on Audible, and you can download them at will. Plus, the app provides all the bookmarking features you'd expect from a professional application. Unfortunately, if your audiobooks are from somewhere other than Audible, you need something a little more flexible.
For non-DRM audiobooks, there are a few stand-out apps. Mort Player and Audiobook Player 2 are the standbys I've been using for a couple years, but the newer Smart Audiobook Player is truly an amazing piece of software. Although it boasts the same features you'd expect from any audiobook player, Smart Audiobook Player also includes:
Support for almost every audio format, including .m4b (the format iPods use).
Built-in cover art searching and downloading.
Lock screen feature to avoid accidental chapter skipping.
Playback speed adjustment.
Audiobooks are organized by putting each book, whether it is a single large file or many small files, into its own folder. Smart Audiobook Player treats each folder as a separate book and sorts the files inside each folder by filename. In order to keep audiobook files from appearing in your music collection, a simple .nomedia file can be added to your root audiobook folder.
Although the features all work together to make an incredible audiobook player, by far my favorite feature is the speed control. By setting playback speed to 1.2x, the voices are still quite comprehensible, and you can cram more book into each morning commute. Smart Audiobook Player is free, but for a $2 in-app purchase, you can unlock the "Full" features permanently, allowing for bookmarking of several books simultaneously, and a few other nifty features. If you listen to audiobooks, but don't purchase them all directly from Audible, you owe it to yourself to try Smart Audiobook Player: http://is.gd/smartaudiobook.