In case you want to take it a step further, you could convert the `mp3`s to `m4b` format (one file per audio book), which internally uses an `mp4` container aac codec, chapters for separation and extended metadata.
It's well supported on iTunes / iPods / iPhone as well as Jellyfin, Kodi, Plex, etc. and the files have lower bitrates than mp3 providing similar quality. (see https://github.com/seanap/Plex-Audiobook-Guide#optional-auto...)
There is also https://www.thorsten-voice.de/en/motivation-vision-english/ (see my comment below), if you would like to generate your own german audio books via TTS and AI.
A self hosted audio book server is audiobookshelf (https://www.audiobookshelf.org/) which comes with an app that is open souce and supports offline listening.
Tools to convert `mp3` to `m4b` and edit metadata:
- AudioBookConverter - https://github.com/yermak/AudioBookConverter (GUI, cross platform)
- Chapter and Verse - http://lodensoftware.com/chapter-and-verse/
- m4b-tool - https://github.com/sandreas/m4b-tool (command line, cross platform)
- tone - https://github.com/sandreas/tone (command line, cross platform)
Note: I'm the author of the last two tools.