Does the book show up in the Books library in iTunes? (select Books under Library top left column)
If yes, you can try the following.
This is supposed to work for some other types of errors. With your device connected, in iTunes select the iPad under the Devices list, then go to the Books tab and turn of AudioBook syncing. Sync. Then turn it back on and sync again. This will remove and then re-add all the audiobooks. This might take a while if you have a lot of long books.