Well now, I just went to the iTunes website.
http://www.apple.com/itunes/working-.../book-faq.html
Does recommended mean required?
Actually it says recommended not required.
If you want one, here is the link to the Bowker site.
https://www.myidentifiers.com
They are not cheap.
Now for anyone else reading this: it is always best to go to the source rather than Henry's blog.