Although I store all audio books as MP3 files, it is sometimes desirable to have them as M4B files. The advantages are that iTunes, for example, can handle this format better. In addition, the M4B format offers the option of setting bookmarks and storing the position in the audio book itself. At GitLab I have deposited a small tool that allows the conversion.
In the „mp32m4b_gui.py“ program (Phyton 3.8 must be installed from the MS Store), specify a directory in which the MP3 files are located. You can specify the quality and the output directory there. The resulting M4B audio book can be split up automatically, because iTunes (and the music app on the iPhone) can only play audio books up to approx. 13 hours long.

Have fun with it