If you have audio books DRM free in an ancient format like MP3 you can convert them using the awesome FFMPEG open source utility.
Here is how I did it for me without using any crapware shared as free (which are not at the best) and ultimately using the same utility in the background.
ffmpeg -i AwesomeYou.mp3 -c:a aac -vn AwesomeYou.m4a
The -vn option allows the transcoding skipping the artwork that often is embedded within the file you are converting.