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I am basing my response on an example offered on the schema.org site. In the example below,the "range" itemprop was set multiple times for the same item. It seems that is how the author situation would be handled. With respect to the isbn, I would assign both. This is a new system and the above are my best guesses based on what I have seen. I am certain these questions will be addressed in detail quite soon. If I was coding today, the above is how I would proceed. If it is later found to be incorrect, it should be easy to remove the extra line of code.