I recently noticed that half my audiobook files have lost their track info! I use that to keep track of whether a file is part 1, 2, etc. So I need it back.
If I get info on an audiobook (from Audible) file in iTunes, the Track and Track Of fields are empty. I fill them in, click OK. Sometimes it shows up there in the Track # column, sometimes not. If I then open Info for that file again, the Track and Track Of fields are blank again!