The metadata is showing what the publisher put into the metadata fields of the book itself. There is no way to edit this as it is deep inside the book. If you really need this fixed, addressing the publisher is the first step, but I think you might as well just let it be, the publishers are not going to completely redo an eBook to add the middle name of the author.
Actually I am the publisher/developer. And the issue is the Nook for Mac is not displaying the author's name correctly - the Metadata DOES have the full name in it, but it is not showing it all, it is truncated.
I was hoping an Admin here or someone from B&N with access to their bug database could tell me if there was a work-around or if this is a Known issue that cannot be fixed.
Here is the metadata info from the content.opf file:
<dc:creator opf:file-as="Lastname, First Middle" opf:role="aut">First Middle Lastname</dc:creator>
Nook for Mac's right top running headers only displays: