I use Nokia Audiobook Player on my N95i. Excellent stuff. It reduces the MP3 files to one third in size (My whole 'In Search of Lost Time' to a mere 250 mb!), keeps record of the farthest listened position of each and every audiobook, keeps bookmarks. What else do you want, blood? Connects seamlessly with my car audio, thanks to a front AUX and the car charger takes the battery-related concerns out of the equation. I only use mobile phones for audiobooks so there are no phone calls that could disrupt the story:
http://www.nokia.com/A4632325