multiple books format in one page

Hi,
I'm new to this list. My name is Gregorio and I work for a publishing house
in Italy.

I was looking for a solution to markup (using Schema.org) our website in
which we sell paper books and ebooks. The fact is that in one single page
(that represent the book) there are both paper and digital version.

I've found in the list archive this topic:
http://lists.w3.org/Archives/Public/public-vocabs/2012Feb/0091.html

I've followed the instructions (example on webpage
https://editrice.effata.it/libro/9788869290428/tolkien-e-i-classici-ebook/),
but the Google's Structured Data Testing Tool says that "Book could not be
a valid target for itemOffered".

Any suggestion?

-- 
Gregorio Pellegrino

Effatà Editrice
T 0121 353452
F 0121 353839
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