I have to admit I don't quite understand what you are trying to do. Do you want the book on your iPad? Because I don't think dropping it on iBooks on your computer will do that.
I tried to add a book to my iPad from iTunes but couldn't find a way, so maybe you are right that this functionality has disappeared, although it seems weird. Nothing I tried seemed to work. I think the simplest way is to either mail yourself the book, or upload it to Google Drive or Dropbox, then open it on the corresponding app on your iPad and use Open in to transfer it to iBooks.
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