Well, if you look at the APA Style, I doubt you'll find the ISBN to be part of any recognised reference. It seems to me you're trying to cobble together something that doesn't conform to the APA Style and instead want to something of your own.

Do note that, when you change the referencing Style, Word changes it throughout the document and the referencing formats Word uses aren't fully compliant with some Styles.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]