I downloaded a mp3 file from author's site (not itune store or audible), and I found that the book is not in iTune's Library/Books section, but in iTune as a song.
What is necessary to make the mp3 ebook is listed in itune's Library/Books section?
Right click the book in iTunes, and select Get Info (or select the track and Press Cmd-I)
iTunes will open the Info for the file. Goto the Options tab and change the Media Kind of the file from Music to Audio Book. It will then appear in the right place.