I put all my 'books' into the 'Albums' tab and let MSP manage the files. Some of the books are single multi-song pdf's and some are separated into single songs. I have the file storage location on a SD card. Keeps everything simple for me and I can't see any advantage in managing the files myself.
Dell Latitude 13.5" 2-in-1 Ubuntu/Win 11
Samsung Note Pro SM-P900 12.2 Android 5.0.2
Samsung S7+, Android 12
Samsung Note Pro SM-P900 12.2 Android 5.0.2
Samsung S7+, Android 12