Look through the iPod's Artist, Album and Songs to see if you can find them under these sections. Could be a tagging issue, right click the audiobooks in iTunes then choose Get Info. Ensure the Name, Artist and Album fields are filled out in the Info tab. You can set the Genre to Audiobook or Spoken Word, but it is not necessary.
64GB iPad 3, 32GB iPhone 4, 1G 16GB touch, 4G & 6G Silver 16GB nano, 5G 60GB iPod, 1G AppleTV, 2G ATV, 2.5GHz 17" MacBook Pro