I'd like to suggest an audio player optimized for audio books -- saves your place, has bookmarks, maybe even integrated note taking, etc. There are some good ones out there but they lack one important, even key, feature. They do not support the speex format (.spx.) This format is specifically designed for the spoken human voice. Although lossy it clearly preserves spoken characteristics. The files are much smaller than corresponding mp3s and (this is a key for me) you can associate the extension. spx to your audio book player instead of your music player and keep things separate. I recently had a glitch where my phone picked a random "song" file as its ringer. The next call rang "Chapter 12 ..." This wouldn't happen if all my books weren't considered music.
One more thing: it would be great it the player or another app if need be could batch convert my mp3 books to spx. There doesn't seem to be an android app to do this.
Please help or pass this idea on to someone who will.