Books mistaken for Audiobooks

So I have this library of books - nearly all of them bought via third parties and all of them being either in ePub or PDF-format. Of course do I edit the book's metadata to sort the lib according to my liking.

Recently, I then made a strange discovery - some books were not syncronized anymore.

But I then noticed that all of my books were still in iBooks, just the sync-process would not work properly anymore.

Researching the issue then resulted in the discovery that iTunes is mistakenly taking some of my books for Audiobooks, when they in fact are just plain / normal ePub / PDF-books and never intended to be heard as Audiobooks. So when I allow iTunes to also sync Audiobooks, would they show up on my iPad, but were empty or could not be heard. Further investigations then led to the point that each time I want to sync my books with my iPad and get proper results, I have to go on each and every item in the Audiobooks section in iTunes, right-click the item and then "Get Info". iTunes then notices the item as a normal book instead of an Audiobook and moves it to the book section. So I have to do this for about 70 items and can afterwards sync normally with my iPad.

Anyone out there have the same issue? Any solution to get the books metadata fixed and get away from "Get Info" to make iTunes aware that these are normal books?