This is because the "Content type" has not been set correctly
For the iPod to identify an mp3 as an audiobook "Content type" must be set to "Audiobook" or "audiobook"
"Content type" is stored in the mp3's id3 tag, as "TCON" so you need to set TCON=audiobook
id3v2 is a program that can modify mp3 id3 tags easily
id3v2—TCON "audiobook" *.mp3