[OptionOfT]

Tangentially related: I like to leave my phone at home when I go exercise, and just listen to books via my watch (21st century problems...)

But to this date I cannot use Apple's Books app on the watch to listen to audiobooks I have on mp3/mp4a/... It only works with audiobooks you have purchased in their walled garden.

[C-Loftus]

Yeah unfortunately I am not sure about the Apple Watch. On iOS I personally use BookPlayer [0] and find it easy to transfer mp3 files via USB. I think there are cloud sync options as well. Been very happy with that if you are looking for other mobile options.

[0] https://apps.apple.com/us/app/bookplayer/id1138219998