Smart Audiobook Player is an excellent app for Android. It is free. No ads. No bothersome popups trying to get you to pay anything. You can pay $2.00 to add a few extra features that you probably don't really need (an equalizer, speed adjustment, etc.) It is worth paying this small amount just to support the developer, as this is really an excellent app.
This is a great app if you already have audiobooks as MP3 files. Just transfer those to your device and Smart Audiobook Player will play them. Many of the audiobook apps tie you into their specific store so that you have to buy your audiobooks from them (e.g., Audible). Smart Audiobook Player does not tie you in like this however.
https://play.google.com/store/apps/d...layer&hl=en_US