Audiobooks are AAC files with the .m4b extension.
The best way to get an .mp3 into the audiobook section is to right click on it in your iTunes Music library, convert it to AAC, go to your file explorer and change the extension from .m4a to .m4b, and then import it back into iTunes. It should automatically go into the Audiobook section. You're doing some lossy re-encoding, but it's an audiobook, who cares.
Tommy Tallarico
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