I use
MortPlayer Audio Books on my Android...Its free, no ads and it works very well with mp3 audiobooks (and more). it looks at folders, plays in order and saves your last location. On top of that it allows you to save other bookmarks manually. Love it, all i use. OT: Though since I got my galaxy s4 i am torn when it comes to music. Mortplayer also has a music player, but I cannot use voice command with it like i can the galaxy player so I flip back and forth between the two.