I also use Listen Audiobook Player [1]. Loaded with well thought out features. The app also saves your progress/stats in a json file in your audiobooks directory, so you can easier move it to a new phone/tablet whenever you want.
I really like Smart Audio Book Player. It is freemium. I think I don't need any the premium features, but I liked so much the usability of the app that I paid for it.
On iOS, searching by title in Overcast's turns up Loyal Books' index of free written and audio books, which seems to have the RSS feeds for the LibriVox audio books.
https://f-droid.org/en/packages/de.ph1b.audiobook/