I ran into this, for instance, with book ISBN #0415585775: There were no listed "authors," I guess because it only had editors, so the $author variable was blank. I'd suggest changing that line of amazon-book-item-details.tpl.php to something like:

<?php if (!empty($author)) { print $author; } elseif (!empty($creator)) { print $creator; } ?>

Note there's also a $participants variable; the above could be extended to print that variable if $creator didn't exist (I'm not sure if that would ever happen).

If this seems amenable, I can create a patch.