I have a Sansa Clip Zip as my go to mp3/audiobook player. It's running Rockbox and you can switch between it and the original firmware on boot.
As I never listen to more than one audiobook at a time I have never needed to worry about bookmarking because it saves the place you were at when you switch off the player.
The Clip Zip is readily available on ebay and amazon for a reasonable cost.
The microSD card slot is a huge selling point for these, as is the sound quality. I wouldn't know what to change to if I had to swap to a different player. For the price, there's nothing else out there remotely as good...IMO
